Best bet is to buy a complete engine from a junkyard...
This way you will have all the nickel & dime parts that will drive you crazy trying to find otherwse...
The 4.3 is identical to a 5.7 internally, less 2 cylinders...
They share the same bore/stroke combination, same pistons, rods, valvetrain, etc...
I'm not sure if the 4.3 is externally balanced or not...
If it's internally balanced, the flywheel should work fine...
The 350's are all internally balanced, and use a neutral balance flywheel...
'86 and newer V8's use a 1-piece rear main seal, and therefore use a different flywheel...
Pretty easy to spot...
TBI units are larger on the 350, so you'll want to use the right one...
Wiring harness should work, as long as you use the ECM for a 350...
4.3 motor mounts should bolt right on a 350, trans will bolt right up too...
There is no difference in the length of the motors from the trans to the motor mounts...
The difference in length is all up front...
You will need a shorter fan shroud, and a thicker radiator with more cooling capacity...
If you're just boppin around town, and not towing, the 4.3 radiator will work fine...
Automatic trannys make alot of heat, not stick trannys...
All the clutch linkage, throttle cable, and anything near the firewall, will be a direct fit...
Like I said, they are identical in every way from the motor mounts back...
You'll obviously need new exhaust manifolds & Y-pipe...
Distributor will need to be for a V8...
Radiator hoses you can shorten, air cleaner you can re-use...
All the fuel lines will match up fine...
Wires & harness to the TBI unit & engine sensors should be the same...
Just be sure to change the ECM (Engine Control Module) aka "Computer"
Biggest word of advice is to find a complete engine from a wrecked vehicle of the same year...
It will be a piece of cake to swap in the 350...
GOOD LUCK !!!
ART