Stock 1972 340 intake and Thermoquad HP limits..?

factory 4 bbl manifold and you cannot do better than a tq (well sometimes on a race only deal) unless you have a lot of money in the X or J heads
if you want to stay stock looking and have not spent money on the J's (guides, hard seats, porting) then start with the Enginequest or 88-92 208 model heads=Magnum heads can also be used with stock manifold with some work and they also flow better- (lots of vendors for the EQ iron heads under several names and various results of work
get your rockers figured out b4 doing camshaft iron or roller rockers- with any duration or lift you are looking at shimming your stands so be prepared to take the time to do it right if roller rockers think 1.6 (or more)
use AMC solid lifters and oil through the pushrods, use 3/8 pushrods (pushrod flex is a problem with long pushrod motors)
once you get the heads figured out (and head flows, and exhaust) fill this out for SFT Street Performance | Jones Cams
get your owner to be real clear on what rpm range he will be happy with, power brakes, gears, etc