It may not help you much but my neighbor did a 2500HD with a '68 GMC cab swap project.
He has a short thread on here under the user name Stryker67. I believe it is titled Frankentruck HD.
He also has YouTube videos of many parts of the build under the username JoDaddy's Garage. He has a TON of restoration how-to videos as well but the Frankentruck HD is in there too.
https://www.youtube.com/watch?v=OWv9rZhpTrU
Anyway, I hope some of that helps!