Andrew

There was alot of good advise there.
I switched out my 3.50 up to 3.0, My truck has the 352 so it has the torque to get her rolling with no problem, I still don't use my granny gear. You would most likely have to with a six cylinder.
The swap is easy and can be done in half a day. The third members can be had for cheep at your local salvage yard. Mine came from a 70's LTD. I still have the old 3.50 pumkin and if I had to could just through it right back in.

My RPM's dropped about 300 and gas mileage went up by quite a bit.

Tranny swap would be alot harder in my opinion.