I'd move the engine back as far as practical, or cab forward, however you're going about it.

Consider lifting the cab a few inches to make room for transmission at the firewall, I raised my 48 cab 3" (or was it 4"?) to help with this.

Also consider how the transmission would be removed for service/repairs in the truck. It'd be poor planning to have to later hack the floor or remove the cab, or remove entire front clip followed by engine and transmission just to get to the torque converter. So you might need to cut the floor and extend the transmission cover up the firewall in any case?

•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-