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DI, I drove a stone stock Dart Lite for about 10 years, and getting out of its way in traffic was definitely NOT an issue at all! It was quicker that my prior 73 /6 with the floor shifted 3 speed in all situations. It would go 85 mph up in 3rd gear, Christainsbug Mtn no problem (an old, steep, straight road here). The lighter weight made all the difference in the performance, despite the lean carb. We must have had different expectations......?? Interesting on your mods and the results But changing the cam and rear gear as you did and the lower mileage results should come as no surprise.
I own 2 1/2 Feather Dusters for the last 17 years and my parents bought one new, and I drove it to school when I turned 16...but...compared to my mom's 3 on the tree 1968 Valiant...the feather would loose the heads up race at the light against it, and Dad's stock Feather tended to need much more room to merge into busy in-town traffic on a good day...it got great mileage and we took it to the Cascades and back almost yearly (lots of altitude gain, not so straight roads..Dad typically got 32 on the flats, 24 uphill at 40 mph in 2nd on the steep grade, and 36 running MOD on the return trip down the pass), and the poor thing would get lapped on all but the down hill run unless it was a VW bug... 
...so I've been around and driving this platform for a quite a spell, stock or other wise...not trying to preach to the choir but having autopsied a stock car I know all the ins and outs of the build and what the intent was when the engineers pulled their last tricks out of the book ...during the final days of the A-body, not all Feathers and Lites got all their special parts out of the factory...the early cars can be 100% (the very early Feathers around August/Sept 1975 build dates...were drum brakes with a 3 speed floor shift cars...oddly enough), panels were all the rave on weight savings...not too long ago I went through my stash of fill necks because all 1967-1976 A-body gas tanks have the same shape and dimensions...the question arose on why a '67 had a fill capacity of 18 gallons, the 70's had a 16 gallon capacity and the Feather had a 14 gallon fill...the answer lies in the tongue position in the fill tube...a neat trick that save the Feather/Lite 2 gallons weight by limiting the fill early...(I put the 1974 fill neck in my Feather and I can get 16 gallons into it now...), so they didn't hang their hat on one big change to cure it all, it's all about a little bit here and there to make it all work out.The carb isn't lean they come stock with a 612 jet..and is plenty fine considering that the 1974 stock Holley 1945's came with a #58 and those are lean (once the end user ups it to a #60 or #61, the 1974 suddenly stops being a slug...LOL). Most of the lack of "performance" is the EGR doing it's job at part throttle after warm up and lowering the volume in the cylinders with inert/unburnable gasses, which also cools them and lowers Nox emissions and supressed ping....so you might have a "200" if everything is working properly.
Dart Lites aren't that light as the added Dart sheet metal makes them a little heavier than the comparable duster...If I added an average 8th grader to the passenger seat in a feather duster the weights would be even between the brands, but they did fine and are actually rarer than the Feather since they only made 1/2 as many.
Lower mileage was no suprise on the mods, but doubling the HP and adding 30% to the already generous torque curve and still hauling down better mileage than most standard stock 225 equipped cars...the point was to illustrate for the original poster what he can expect with his build if he steps up to various levels of street so he can make an informed decision based on real life experience and put together a balanced package that will do what he desires.
All dyno data across the board and the engineers know that advancing the cam adds low to mid range torque , retarding the cam loses that torque but makes the build work better at top rpms... Narrow LSA (low numbers) increases maximum torque, moves best torque to a lower rpm band....Widening the LSA(high numbers) raises peak torque to a higher rpm, reduces max torque, broadens the power band (which in a smog regulation laden car, is needed so it functions well from off idle to passing rpm), and improves low rpm vacuum readings and improves idle quality... (in the 'good old 70's days', Ford actually set the cam in the 460's retarded 10 degrees to keep it on the street in both the trucks and cars... hoping that the band-aid would allow the end user to pass smog, and hoping the end user would not notice the decrease in seat of the pants 'umph'). When posting it's always good to cover a fair amount of scenarios so there is much food for thought, I am just lucky to have worked through a fair amount to my duster and a few other A-bodies for typical street use and some for a lot more performance.
There are a fair amount of very good articles on cam basics, dynamics and theory which all mirror the same characteristics and data in their end results...some have distilled all the results down to a good table and make for good reading for up and coming engine builders... as always there is a compromise...when you put your foot down on one end of the scale or the other in configuration, advance, etc....something will be a give or take...more power for a little less mileage...less low end grunt for better idle and even power over a wider range of rpm...
Back to the original post: What Feather_Duster wants out of the end result will be determined by staying with the 1 barrel or upgrading to a 2...and if he wants to retain the stock mileage or sacrifice a bit of that 28-30 something mpg for mid-20's and some nice low end gump..
'nuff said, let's get back to helping our member figure out what he expects or desires out of the build...
-D.Idiot
. Overdrive was practically not usable - car would chug too much, even on the interstate. I have a 3.23 rear end in it. I would like to keep at least 25 mpg. I am interested in low rpm torque and mileage. 
