Firewall question for LS swap guy

[Law79]

Hey gents. I am about to start the cab body work on my 69. I plan on shaving the firewall and would like to know if there is anything in particular I should be doing. Currently the frame is PB Extreme dropmember and I still have a perfectly good running 350/350 setup. I just want to do the mods for the LS swap just in case my engine craps out on me. Not really wanting to do body work twice if I don't have to.

Also I would like to see if anyone knows of a good fuel cell that is set up for carb with the efi option. I like this one but don't know if it'll support what I'm trying to do.
http://www.fueltankparts.com/67-72-e...side-fill.html

Thanks in advance for you help