[gmc684x4]

Balancers are interchangeable unless your old engine is a 400

Acc bolt holes will be the same
No acc bolt holes in heads predate 71 truck

305 should be tbi intake bolt pattern
If swapping to make it carbrated look for tbi carb intake

Pre 86 engines are 2 peice rear main seal
Post 86 engines are 1 peice rear main seal

Flywheel can be re used if auto with correct starter
Manual trucks use 168 tooth flywheels and there starter bolts to the bell housing

Yes vortec heads acc bolt holes are still standard thread bolt heads

You will need to change temperature sending unit out as the one from the 86 will not read right in your truck

Depending on what position you might not be able to use long water pump setup with the engine in 2 wheel drive stock position