Welcome to the forum!
It depends on what your trying to do. For general cruising about, the BBD is an ideal setup. Some mods can be made that will help performance there too. If your looking for bling factor, the 390 holley can be made to work on an otherwise stock engine. It won't perform as well on a stock engine as the BBD, but set up properly can deliver better cross country cruise economy. Not so much around town or for combined mileage. Fuel economy will tank around town no matter what you do. Stop and go will have you in the enrichment circuit unless your on the idle circuit waiting on a light.
Generally, your going to want more compression. Take a look around the Engine FAQ. There is an Engine Build Matrix.
To take advantage of a 4 bbl, you'll want to bump compression up to at least 8.5:1 Static. That's pretty much minimum, and it isn't going to improve your 1/4 mile time much, if at all. Your timing curve isn't necessarily ideal to begin with, and when working toward better power, compression needs to be on the top of your to-do list for a 4 bbl.
Flow being what it is, minimum head pipe size is a 2-1/4" head pipe for the OEM manifold, but that helps even with a 1 bbl. To build on that, a modification can be made by installing Dutra Duals, giving you two head pipes. The new casting front manifold coupled with a modified rear maintains heat for the plenum for good drive-ability. If you are building power where you need dual manifolds or headers, your target compression should be 9:1 or greater. You can keep the tail pipe size with no compromise, as the gasses will have cooled after the muffler, and running a large tail pipe will result in low exhaust gas velocity, which costs you in performance and can result in early muffler rot. It takes hot gasses a while to get water out of the exhaust system.
When you get out to big exhaust, installing oversize valves will help, along with careful port and bowl work. I say careful, since there are water passages that limit what you can do. Making the ports bigger in these heads doesn't help a lot. Cleaning them up does. The head is designed to flow for a smaller engine. The 170. While there are different heads, they aren't designed to flow well for larger displacement. (Like the 225) The problem with hogging the ports out is that fuel air velocity in the port can't be maintained for effective scavenging effect. As the engine displacement increases, volumetric efficiency decreases due to the limitations of the head. While increasing engine displacement helps with power, diminishing returns are realized almost immediately.
Do a lot of reading here, and hopefully there are still a few photos floating about that can be enlightening when it comes to head work. Don't put over-size seats in. The machiniest will need to cut dangerously close to water.
Things that can help with combined efficiency for acceleration are, The rear gear ratio, torque converter stall if applicable, and full mechanical timing advance. Cam selection, and parasitic loads get to play their part as well. Things like power steering. Changing out to a manual box was worth a tenth most passes on my time slip.
Just a few random targets. Don't forget the brakes if you haven't already addressed that aspect. My first stop from high speed on the stock drum brakes might as well have been driving on wet ice. Good thing there's a 1/4 mile to slow down after the stripe. There are kits for disc brakes out there, and there are various Mopar disc brake setups that work well. A good search of the site can lead you to a much safer situation. I installed manual discs from a V8 A-Body, improving braking phenomenally.
Enjoy the journey!
CJ
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