GT class will test your wallet. Spend a lot of time reading the class guides and choose accordingly. Just a guess, but your best bet will probably be a later model 4bbl 318 or 360.

For Stock, change the grille, claim it as a '74 and run a 360 4bbl. You might also need to change the hood and decklid, but I doubt anyone will care unless you go out and run 1.2 under.

I know that NHRA pulls the VIN so if I will need a "donor vin" to change from a "feather duster" to a regular duster. If I can do that then I can run a 318 and run O/SA.

That combo is a turd.

I believe a Feather Duster is about 2950 lbs

The VIN on a Mopar does not differentiate between the feather duster and regular duster, the fender tag, the call out on the front panels, and the aluminum parts make that happen...

To make a complete change to another year, or "clone" a duster, you have to change the VIN tag on the dash, the fender tag under the hood, the partial VIN stamp on the top of the radiator support,
the ID/VIN/Federal Sticker on the drivers side door (above the latch mechanism), there also is a concealed VIN stamp on the floor pan reinforcement under the back seat/carpet. If changing engines you will need to change the vacuum line routing sticker on the fender, if changing years, you may also need to alter the partial VIN stamp on the engine block pad and transmission pad...If to a year previous to 1975, the Catalytic Convertor sticker will need to be removed...(and if an inspector is really savvy, the instrument cluster will need to change to a non Check EGR pre 1974 model...)

Be aware that if you insure the car and get caught with a mismatch, things could get really interesting...
Yeah, there are lighter better mopars to run on the 6 cylinder...been there drove that...
That kind of depends on the options, the automatic transmission will be a bit heavier than the A-833OD combination...FYI.

get rid of the aluminum bumper mounts (and other feather duster items)

The "Feather/Lite" body parts are hood, deck lid, front and rear bumper reinforcement... Hood can be got from 1973-1976 Valiant, Decklid from 1970-1976 Duster/Demon/Dart Sport,
bumpers will be from standard model Duster 1974-1976 with the shock style mounts...the shocks weight more than aluminum bumper...

I have 2 1/2 Feather Dusters and empty they weigh about 2800lbs with the aluminum A-833OD (NHRA is close for rule purposes)...once you add 14 gallons of gas, the driver, etc...
also the Space Duster option will weigh more than the standard backseat by a few pounds, bench seat a few pounds more than the bucket seat options...

FYI.