Accelerator pump issues

[Chevyman13]

Hello, I am new to this forum and have a question that I am hoping someone has ran accross before. We have put a new GM crate 350 in a 1970 c10, owner wanted it kept stock, so we rebuilt the carb, cleaned out the manifold and re-used them from the original 307. Works great for about 3 days and then the truck starts bogging down on acceleration unless you very slowly apply gas. We are finding the accelerator pump rubber is swelling up and causing a lack of fuel on accel. We found a company that says this is fairly common with todays ethenol fuel. They had a "Garter spring" equipped pump that was designed to solve this issue. We bought one and sure enough, 3 days later same thing. Any ideas? Anyone have a source for the original style leather puimps? Any help would be appreciated.