Pretty straightforward:
1) Replace the efi manifold with the carb manifold, GM even offers one or two models in their GMPP catolog.
2) Drop in an HEI distributor (do not forget to remove the LT1's oil pump drive assembly)., and either leave the optispark hanging up front, or get an opti eliminator bracket from GMPP.
3) Run a regualr electric fuel pump.
4) Remember that the LT1 is a reverse coolant flow engine, so connect the radiator hoses accordingly.
Its been a while since I looked over an Impy LT1, but if the fuel pump bosses are there they could be machined to use a regular mechanical pump - engine disassembly required! Easier just to go electric on the fuel pump.
Thomas