10.3 for 71 down from 10.5, but not sure how that was reached maybe the j heads have a couple more cc's? My early 71 intake has no jets in the floor, also remember reading the auto trans having a higher shift point due to a governor change??

Properly dialed-in, my money's on the '71...

X heads may be a tad better than a J head

Maybe but I think most 71's had the j's

X head flows a little better than the later J head, just by a smidge. The X was designed as a 2.02 head, the J a 1.88. They put 2.02 valves in the J's for 71 but you know what that does to the short turn. If you had the choice of the two you would choose the X head for 1970

wouldn't it have been nice in 1971 if Ma used the 69 4-seed cam with the X heads and the 70 exhaust manifolds with the 71 t-quad and intake.. or 6 pack

By the way, J heads were phased in during 1970 model year,and not just on AAR's and T/A's. They had 2.02 intakes etc. Some argue that the J's flow marginally better, but from what I have seen it is a wash.

The earliest 340 4bbl J heads that I have seen are dated October 1969. I am always looking for older dates, I have been trying to figure out when they started using them in 1970. If anyone has pictures of earlier non -T/A J heads please send me a picture. Thanks.

What about the 1968 340 four speed? From what I remember they had a bit different cam and thus better performance.

68 4sp had a lumpier camshaft. Maybe a 4sp 68 notch would be the hot ticket.