I'm looking to swap out my 350 out of my 86 Gmc k2500 with a 350 from a 1975 k2500. Mine is a manual transmission whereas the 75 is a automatic. Will it be directly compatible?

 

Yes if your only swapping engines, course you will need to keep flywheel, clutch an pressure plate from 86. If you want swap in the auto trans, you will need trans cooling lines and stuff, and check the length of trans and slline output shaft ,
as drive shafts might be different

 

The 75 version of the LT-1 ultimately succumbed to new emissions standards that brought its horsepower all the way down to 145 HP. The LT-1 was the last and arguably greatest of the old fuel-guzzling, emissions-spewing 350 engines. Those original LT-1s from 1970 will go down as some of the best engines of their time