Actually, you left out #1 problem with LS4, it lacks provisions for starter mounting. Your other concerns are rather trivial, comparing to everything else that has to be done during a motor swap.

I have seen them as low as $500-$700. This one was 2 miles from my home:

http://pages.ebay.com/link/?nav=item...d=181595245407

I am not sure if stock 3800 offers enough performance increase to warrant to be considered for a swap. You should be able to add force induction to 3.5 with similar results.