To complicate matters, an AD 3/4 ton and larger has a 36" wide frame. Some later axles still interchange, and those that won't it's not hard to remove and position the spring pads where you need them.

Then later on as you say 34" is pretty much standard for all bigger trucks.

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-