Hoping I understood all that correctly: if'n it was me, and it ain't, I'd use the stock Townie frame, as is. The springs are all the same width between all trucks, and the same length. Should be a direct bolt up. Unless you are going to be ultra-airborn 4 wheeling, which is pretty hard with a D-200 2WD drivetrain, the stock Townie mounts are plenty strong enough.

Looks can be deceiving. I know for a fact that the steel used in the earlier trucks is much better quality. Metal's spark color is a reliable and metalugically acceptable test of a metal's hardness & carbon content. Grinding on them you'll notice bright blue sparks off the older frames, and yellowish sparks off the newer! Much less carbon, so a softer metal, proven by spark color. Dodge knew that, so they made the metal heavier in newer frame parts, because metal itself wasn't as strong as earlier frame parts!

Dodge strongly cautions against flame cutting or welding on the early frames because the metal is heat treated, tempered! If you MUST cut & weld then it's necessary to fish-plate, or run reinforcement plates, or box the frame, at least 8" either side of the weld.

Personally I'd use the stock frame and what appear to be lighter spring mounts. They should work fine for heavy duty use. However the front spring forward-mounted spring shackles sometimes need some large flat washers added to beef up the mounting. You can also weld on large heavy flat washers where the bolts run through the solid spring mounts.

Should work just great and save you a whole bunch of work & expense. Just a $.02 opinion.....