Then Mike just answered your question about using the back half of a newer truck frame.
Unless you make a funny looking transition from 36" down to 34" it won't work.

Although I suppose you could add the new frame about mid way under the cab, where the frame tapers in and at some point in that taper it is 34". You would have to adapt the rear cab mounts.

Probably simpler and less work and time to find a decent used frame for the model truck you already have....

Grigg


1951 GMC 250 in the Project Journals
1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-