I'm not sure if there are any differences between the 4wd and 2wd frames. Best bet would be to try to locate a 2WD 87 or newer Ram 50 and compare the frames. If the frames are the same I'd find a doner 2wd and transfer the 2wd front suspension pieces. If not it will probably be a lot simpler and cheaper in the long run to just swap out the whole frame from to a 2wd version.

For a variety of reasons I would definitely NOT use the Mustang II front suspension.

Depending on what you want to do with the truck you probably also want to keep in mind the stock rear end is probably geared pretty low (3.54 or 3.90) If you are planning on doing much driving with it especially if you stay with relatively short tires a rear end swap or OD transmission might be in order.

My daily driver is a 2wd 87 Ram 50 with a small block and 5 speed. I'm using stock front and rear suspension and it works great....basically it's a ball to drive and I've even taken it back to the mid west a few times.