The PO built the engine mounts on mine so this info may not help you but here goes, my LS1 came out of a 98 and the oil pan works fine, alt clears fine, but the AC had to be moved and I replaced the factory compressor with a Sanden 501 (I think), there is a company that makes a nice Compressor relocation kit for the sanden.
http://www.kwikperf.com/lsx_ac.html I ended up making my own.
Check my build thread for pictures of most of this.
I think if you remove the compressor for now you should be able to set it in the frame, the alt will tuck in the frame rail if it's like mine.
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71 Cheyenne SWB LS1 and auto.
http://67-72chevytrucks.com/vboard/s...d.php?t=256470
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