I would go for PU or PP 5w30 or M1 0w-40.
Your '90 350 is nowhere near the first generation of 350 engines. The first one was introduced in 1967.
You are correct, however the overall design and layout of the engine was the same from 1968 to 1985 at which point they added things like a one piece rear main seal, roller cam shafts and some other minor things. 1967 was the odd ball of the 350's because it used the earlier "small" bearing journals and cartridge oil filters. All of the small-block V8's from 1955 until LT1's were introduced in '92 were refered to as the generation 1 small-blocks, at least by GM. Heck, they continued to use the "1st generation" up through the 2003 model year vans.