I have a 1994 F150 with a 300-6cyl and auto trans and 4whl drive

and want to swap in a 1991 5.0 from a Mustang GT with an auto trans,

is this possible without costing a fortune.

A bit of research on the internet should tell you if the bellhousing bolt patterns are the same between your two engines, if not is there an auto trans from another application than will bolt your V8 to your transfer case?

Lastly before go you and do this see if you can find factory dyno graphs of each engine. You may find the I6 is better matched to your truck they typically produce more torqe over a wider RPM range than a comparable V8. 

The bellhousing is the same as a 5.0. the problem is that the I-6 has a much nicer torque band and is relatively a very strong and reliable engine for a 4x4. and you rear end may be geared  for the I-6

If you intend to change, be ready for bad gas milage.