If the 70 is a short box truck, the 73-93 frame fits very well. the rear frame above the axle on the 73-93 frame kicks up a couple inches, but when we swapped my son's 71, we just cut the top of the kick up off and welded in a piece of 1/8" x 2" wide flat steel bar. The whole swap took about 8 hours in our driveway. Short box wheel base is a match up. The best part of that swap is you still have a truck you can use as a truck if you want to.

The way modern swaps are going, it seems a Durango (after 2004 they put 5.7 Hemis in them) is a good chassis to start with. A lot of the guys building 50s trucks are starting with them, they use everything except the body, and some are using the firewall & front floor pan. The wheel base and firewall location might be off for a 70 though. might be worth doing some measurements. I suspect the full size modern Ram will be too wide. Gene