There are lots of ways to accomplish your goals.
1) Modify your stock sending unit for double pumps.
2) Modify a stock unit with a 1/2", 5/8" or 3/4" pick up and run an external pump.
3) Use a fuel cell
4) Sump a new stock tank
5) Build a dual pump/dual outlet sending unit like this one:
http://www.turbobuick.com/forums/general-turbo-buick-tech/215155-new-double-pumper-sending-unit.html
6) Use larger feed and return lines with a stock rail by building one of these to replace the stock regulator (on a V6, but not sure what would be needed with the 4.8) (WHY a 4.8 when the are the same physical size as a 6.2?.....no replacement for displacement.) Unless you already have a 4.8 and don't have the funds for a 6.0 or 6.2, that I understand.