Lester Breuer asked:
"I was gifted this HO Train-Miniature NP Hopper lettered correctly for the NP and being 35 ft inside inside (rather than 34 ft 9 in as the prototype) it could be reworked into a NP Hopper in the 70050-70199 series which had 6 panels rather than 9 as the AAR standard design with 33 ft inside. Rivets pattern also need correction. I began the process (2nd Photo) and began wondering if there is a better model for the project. Is there a better model for a starting point?"
Probably as good a starting point as any, especially as you've started carving plastic. The Train-Miniature/Walthers 36 ft offset twin is a foobie anyway (they tooled an offset car to get extra mileage out of the tooling done for the PS-3 twin), so you're not wrecking something which would be otherwise useful anyway. The only other longer model twin hopper that comes to mind as a starting point is the Gilbert offset twin (undated catalog scan from the HOSeeker website attached for reference). The linked clearance diagram from the NPRHA website doesn't indicate the type of taper, so I'm not sure if the stepped taper of the Gilbert model or the long taper of the Train-Miniature/Walthers model is more appropriate.
I got one of the Gilbert cars years ago as a basket case and stripped it, then failed to find a prototype for it. I also have a couple of the Train-Miniature offset twins on hand, so this is a potential project for me as well.