first im replacing the center metal ttop piece on the roof and was wondering what do i put under it to seal it from leaking?does it go all the way around it to seal it?

next im replacing the 305tpi with a 383 carb do i have to replace all the fuel line setup(pump setup) since there is 2 lines to the front for return and gas?very much

When I replaced mine,I cut a thin piece of of rubber gasket(to form) and used 3M weatherstripping adhesive under it,working well."Knock on Wood"
Don't know about the fuel line.

thanks

for the fuel lines most people are either adapting there lines to a return style pressure regulator or extending the tube to the fuel pump further and running an engine mounted fuel pump with adapted lines or a low pressure in line electric fuel pump and some adapting of lines.