Just to answer the first question; YES, the 4AC crank can be used in the 4AG block.
So, if you are not in a hurry, don't mind pulling your 4AC to pieces, and just want to
try your hand at building a running engine, you can use a lot of 4AC parts. Both have
the 4A block, and as far as I know the crank, rods, bearings etc are interchangeable.
The only known difference is that the 4AC crank has 6 bolts instead of the 8 bolts
on the 4AG. Just means you have to use the 4AC flywheel.
As said above, it is not good practice to mix and match used engine parts as certain
parts such as bearing caps (rods, mains, camshafts) are custom matched to each
engine and should always go back on the same bearing and facing the same direction
as when the engine left the factory. BUT, if you are not trying to build a race engine,
you can just make sure that everything fits without getting tight or nipping the shaft
and hope for the best
Seeing as the parts are used and worn, you can use as much as you can take from
the 4AC and swap it onto the 4AG block. Go track down any missing parts, and then
see if you can get it to run. Great learning exercise... how to use a torque wrench
and put an engine together without spending a whole bunch of cash.
Cheers... jondee86