This question has probably been answered somewhere else in this thread but I'm going to ask anyway. Is there a direct swap from some other vehical to replace the rear end on a 5th gen Elky. I want a 3.42 that will handle 350+hp with limited slip of course.

grand national

3.42 with limited slip were an option on Monte Carlo SS, Cutlass 442, Grand National, Regal T type. The 7.5 or 7.65 rear will hold up to 350 hp, it's the hard lanching that kills them.
You could always cut the ends of the axle tube off and have the Ford bolt on ends welded in place. There are a few aftermarket places that have these ends.

This to me is the biggest issue with the Fifth Gen. I am in the very same boat and have been kicking around all of the options. My goals are basically the same as you Rock Fly and to me I just don't know if there is anything that is worth doing besides a custom 12 bolt nine inch set up. The option that I have thought about is pulling and eighties posi out of a Camaro or trans am or HO Monte SS these posi units are bit stronger and with the proper modifications (Axles bearing support)can be made to live on the street. My problem is sooner or later I will want to go to the Drag strip and IMHO this would spell death for that set up. I have spoken with a couple of builders and they have offered packages of 10 bolt 12 bolt and nine inch the 10 bolt is adequate and might last on the strip if prepped properly but again if drag racing the obvious choice should be a Nine inch or a 12 bolt but those options for me blow my budget out of the Park.....