Something must be wrong with your 318, either the tune, or its low on compression, i just purchased my 2nd. 71 Swinger 318 car, it is bone stock with the original exhaust (hanging on the ground i might add lol), it has 89000 miles on it, got it running after sitting 3 yrs., i had to install a rinky-dink elec. pump, it has about 3.1/2 lbs. pressure, i will say it has balls being bone stock, i could only amazin how well it would do with an extra 80-100 hp. & some 3.91s, plus a 100 shot to boot. The last 71 318 Swinger i owned would roast the tire for a block, & that one huffed blue smoke 8).
I may have just talked myself into something:-D