A 46 has a pretty snug engine compartment, and a tight cab too.
Is the 6BT a good fit?
Will a 4BT give you the power you want and fit in there a little cleaner with less if any firewall modifications?

I've seen pictures of a few 6BT's swapped in AD trucks and so far I haven't seen an outstanding one, although I bet many of them are functional.

Not to dissuade you, just encouraging some more thought ohwell

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-