There are motor mount relocators/adapters for the LS engine swap, to allow you to mount to older style frame stands.
There are adapter kits to allow attaching the 700R4/non LS version of the 4L60E to the LS engines. Very simple spacer/adapter.
Holley, Trans-Dapt, Summit Racing are just a few who sell the mounts, Summit for the best deal on the trans adapter.
A Chevy truck forum specifically for the '73-87 Model is probably going to be your best bet for info on the swap, as to whether a different oil pan is necessary, wiring, exhaust fit.
The 1973 - 1987 Chevrolet & GMC Squarebody Pickups Message Board - The 1947 - Present Chevrolet & GMC Truck Message Board Network