I am in the process of installing a LS1 and 4l60e transmission in my 1986 El Camino or G Body. I am using John B's motor mounts and notched F-Body (Camaro/firebird) oil pan. Just did a trial fit and it looks like they both work.
The Corvette LS1 and F-Body LS1 are identical except cam, heads, and intake on the Corvette give a little more HP. For example a 2000 LS1 in a Corvette is 345HP and the F-Body is 320HP. The accessories are different and are interchangeable that is with swaping all brackets including the appropriate balancer, in fact the truck accessories will bolt on to any LS1.
Items you need to solve:
I would suggest you purchase a custom engine harness and computer from SpearTech or other harness maker. SpearTech can even set you up with the Corvette drive by wire as that is what I am installing. SpearTech.com
Transmission choice as a 4l60e is electronic and will bolt up to a Corvette LS1 also as it does come in a F-Body or go with manual transmission.
Temp sensor for gauge needs to be connected to passenger head an adapter can be had from S&P www.hotrodlane.cc
since it is 12mm thread or just drill and tap.
Oil pressure sensor needs to be added from original car to the adapter above the oil filter on LS1 just drill and tap. Might need to add fittings to move it away from exhaust manifolds. S&P has an oil tree to help.
Motor mounts S&P has some and there are others but most move the engine up and forward to clear chassy items. This works but will push more weight forward. I chose to use John B's setup as his only moves the engine forward 5/8" other than that it appears to be in the stock location. He has done the measuring for you.
Need to fabricate or purchase a transmission crossmember to fit transmission of choice. I purchased a G-Force for the 4l60e with LS1 that has cut outs for exhaust system. Should be a direct bolt on to my El Camino.
Then there are other smaller items to contend with like fuel system upgrade to Fuel Injection (FI), Gas tank, sender, intank pump for 58PSI. Pressure regulator filter setup, return line. Then the dreaded fuel connection on the fuel rail how to connect, various methods.
John B's website gets you the shoping list. Do your homework and solve all issues before you start or at least have some solutions.
I hope this helps. Back to my installation......