Sounds like a lot of work.

I'm not familiar with the 39 models, but I find it hard to believe that replacing motor mounts is very difficult. This would have been anticipated, and should be easy, certianly not by unriveting a cross member.
What does the shop manual say? Here's the page in 1938 manual http://chevy.oldcarmanualproject.com/shop/1938/38csm157.html


•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
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-