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It all fits under an Air Grabber. I want the appearance to remain largely stock and keep the Air Grabber fully functional, so I don’t anticipate making any changes with the carburetor or intake.

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Not looking to make it a 12 second car, but if I’m taking it apart, I would like to get the performance a little more ...I just want to be able to exert some more authority at the stop light.

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Here are my thoughts:
It's a smallblock RR, and a 72, the orig 340 is long gone, and the current 340 is 'damaged'. 'They' () would say this car is lower in desirability compared to 68-70 B-bodies, so I wouldn't get too hung up on building the engine to preserve its value for a future buyer. Build what you want.

re: Magnum heads...
Changing to Magnum heads would require re-drilling the heads to fit LA intake bolt configuration, so keep that in mind. You could use a Crosswind intake (has Mag and LA bolting provisions), but then orig appearance is definitely gone and Air Grabber fitment is likely to be gone.

For me, I would find a LA360 roller engine, get a Bullet regrind or aftermarket roller cam, and stick the J-heads on top. If you then want a little ‘more’, re-drill Mag heads and use LA covers and your intake/carb.

FYI:
You’d need to do some researching/planning if you anticipate to swap to Mag heads later. Whatever cam you use for LA heads would need a cross-drilled cam journal for rockershaft oiling, then you’d probably want to block that passage for Mag heads (to keep oil downstairs) and change to oil-thru lifters & pushrods. The oil could be blocked with a Mag headgasket, and the LA engine might already have some oil-thru parts in it (according to some stuff I’ve read on here). Patrick () is one of the gurus for Mag vs LA info.

For an LA roller 360, you need a late 80s carb engine or a 90-93 TBI engine. Carb engine has provisions in the block for a roller cam, but has a flatcam. A TBI engine has the roller cam system, but might not have an extended snout for a mech fuel pump (carbed-318 roller cams have the snout, but I don't think the 360 was ever available with roller cam and carb, so I'm speculating ). Hughes Engines sells an extender snout for a TBI cam (cheap). A Magnum engine would have roller parts in it also, but its cam is NOT cross-drilled for use with LA rockershaft oiling.

There are +/- to whether to get a carb or TBI or Magnum engine to start with, so it kinda depends on your ultimate desired combination and what engines/parts you can find to buy.