Buick 300 Stuff
Nothing else fits. I'm not sure if a 4 barel manifold from '64 even works. '65 went to cast iron heads and the intake port flange angle is in question.
Parts and Books
We all need the parts your talking about. The best book is GM's BUICK master parts list of chassis parts. Always over $100 then BUICK's Shop Manual. Motor Manuals from the 50's are good for all cars. You have to be on constant lookout for parts good and worn to stay in the game.
300 Heads
Did your heads drip black oil from the head gaskets and only around the front and back ends? I can't tell if it's from the rocker feeds or old valley slime. Cylinder compression pressure is an even 120# all around with no coolant loss. The engine is from a Skylark with 4 barrel carb...
Fuel Pump
A lot of pumps have already been rebuilt with mods and kits don't work. Try to get a rebuilt pump. Going to electric can't fix a leaker or no wipers.