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It can be done. If the engine was a direct replacement for a '79 truck, you'll have a few things to work around. The '79-earlier engines had the dipstick on the driver side; if you keep you stock exhaust manifolds, the dipstick tube and manifold try to occupy the same space at the same time (headers solves the problem). '85-earlier had a two-piece rear main crank seal, and your '90 has a one-piece - you'll need a different flexplate (not a biggie, just a detail). '86-earlier heads had all of the intake mount bolts at the same 90 degree angle, while '87-later (cast iron) heads have the center two intake mount bolts on each side at 72 degrees. Your '90 TPI base will either have to be replaced with an '85-'86 base, Corvette base, or modify those bolt holes in the base so you can get the bolts in.
Truck heads usually weren't very good from a performance standpoint - small valves (poor flow), large chambers (low compression), crack prone, and generally just don't run well. Your current 305 heads are a better choice. With 1.94" intake valves installed, and assuming your 350 has dished pistons (pretty strong likelihood), better compression, and the above TPI base problems now go away.
You will also either need to get a 350 computer chip, or have your 305 chip reprogrammed for the 350.
You'll also want to get 350 injectors.
Other than that, piece of cake!
