Differences between earlier /6 engines and newer ones

No, sir. The '67-'74 Slant-6s are the "cream of the crop": they have the forged crank (instead of weaker cast starting mid '76), they have the bigger bearings and stouter connecting rods (smaller bearings and narrower rods to go with cast crank starting mid '76), they have the improved combustion chamber (started '67) with gasket-seat spark plugs (more options in those than in the taper-seat plugs that started in '75, plus the '74-down heads weigh less and you can R&R the lifters without removing the head). These blocks are also sturdier than the later ones which were cost-reduced by taking metal out. Hydraulic cam started in '81 (after a large production-engine test run of them in '78)…nice that you don't have to adjust valves, but the stock hydro cam is small/weak/wheezy.

Easy to understand how and why they got away with cheapening and weakening the Slant-6: at the same time they were letting it die a slow death by not updating and upgrading it, just de-tuning it further and further to squeak past tighter and tighter emissions regs. By the sad end, it was wheezing out 85 horsepower...not enough to break even much weaker parts.

If you're going to the trouble to do a performance build, might as well start with the best, strongest parts. If all you need is a motor and you don't care beyond that, then it doesn't much matter. But be aware that in addition to needing to use the oil pan and pickup that goes with the vehicle, the crankshaft counterbore and torque converter nose diameter changed after '67. You can use a '68-up large-counterbore crank/engine in front of a '67-down small-nose torque converter but you must install a ring to take up the space between the TC nose and the crank counterbore or you will eat trans front pump bearings and seals. You cannot put a '67-down small-counterbore crank/engine in front of a '68-up automatic trans unless you have a special torque converter custom-built, or you build up a "FrankenTrans".