Would that be a good swap into a 96 mustang? Ford had a straight six in the 95 f150's, my grandfathers truck has one. Would that be even worth trying to swap? I want the torque of the straight six but if it's too much trouble then i'll leave it alone.
A 4.2 swap is probably the easiest engine swap you can do on a 3.8. The I-6 4.9 in F150's is probably too long to fit in the engine bay.
The 4.2 swap has been covered several times at various V6 mustang messageboards. Do a search on "4.2" at V6power.net, 3.8mustang.com, and here to see what is involved. There are several different ways you can go about it depending on budget, goals, and knowledge.