I have a 72 GMC Sprint w/ a 350, w/ a Holly 4 barrel. When I start it up, it idles fine. After a warm-up period, I put it into drive & it starts to spit & sputter when I hit that gas. I have to give it a lot of gas to start off. It is fine up to about 20 mph, then it wants to cut out. I have replaced the Fuel Filter, Fuel Pump, Starter, & have checked the jets on the Carb. Any help will be most appreciated.