I have a feeling that with a front frame swap the engine and trans will be low enough that floor clearence won't be a problem.
As for firewall you will have to measure center of front wheel to firewall on both vehicles and see for your self.

•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-