Slant 6, 318 or 360?

Go with the 360. That's what I did in my '89 Ramcharger. Getting close to finishing the project. Probably next month sometime. The 89 Ramcharger is exactly the same setup as your '87 Ram, if you have any questions, just ask. I can help.

Don't worry about the rear, it's a 9 1/4. Plenty strong for your 360. Best thing is that it's pretty much a drop-in, except for the driver's engine mount bracket. I got a new one for $25. Other than that, It should bolt right up. \\:D/