Im stumped!!

[Seatown-gmc]

I have a 70 GMC pickup with a factory 396. I have had problems with them fuel delivery since i bought it and i have been trying to figure it out. I think it is vapor lock cause when it starts up cold it runs great but slowly leans way out. it never dies but starts to pop and cough like it is running out of gas. i installed a clear filter to watch and see what it it does. Its doing what i expected the filter starts out full then little by little about 20 minutes time the thing is bone dry. I replaced the fuel pump and also wrapped the line from the pump to the carb with insulation. the truck has headers and the fuel line runs down the frame but the line looks like its not too close to the header. do you think i could have a oven effect with the header and the frame. im lost i dont know the deal is. the truck has three tanks and it does the the same thing on all the tanks. the truck has a edelbrock carb on it do you think that could have anything to do with it.