My 1960 Chevrolet L60 Viking LCF is the same way, I'm looking to replace most of the cab with a better one, but it will be a bit easier then your truck. the front frender don't came back into the doors as far as your do.

I have a spare cab kicking around the place for my project, just need time & a Shop....

1965 GMC 3500 Rescue in the Stovebolt Gallery

My Facebook Links:
Jolly Goodfellow.
the 6066 GMC Guy.
Jolly's Truck & Car Show Page.
Owner of ~ 1965 GMC 3500 Custom Rescue Squad {Black Built Bodies}.