If you want to use the '83 block, you either have to use it 'as is' or change the internals - crank, rods, pistons, harmonic balancer, and flex plate or flywheel using the '68 innards. Any 302W block up to '81 is virtually identical with any other. Post '81 - blocks remain the same, rotating components changed balance.

Weight wise - first I've heard that one. Your buddies might have figured that the 50 in oz vs the 28 in oz balance is the weight difference in the crank - it isn't. They physically weigh the same. You can mix and match between the engines internally, but you will have to do a total rebalance.

Fords, regardless of the source are confusing since most folks seem to only understand Chevies. They really are not any different as far as confusion once you understand that there are year to year changes plus changes to fit an application. For instance, I can take the cylinder head from my '31 and bolt it on your '68 - fits right up. The intake, same thing. Exhaust, ditto. Starter, yep. You get into confusion when you start looking at accessory brackets and front timing covers with water pumps. Balance is another, but as long as you know what you have, even that's no problem.