You're best to measure and go from there. I know guys have used full size GM and Dodge frames, but I don't know of the modification requirements.
The truck is yours though, and the decision of whether or not to modify it is yours alone. What do you want to do with it? Do you want a "parade truck", or do you want something you can use as a heavy duty hauler, towing and hauling, and doing actual freeway speeds?
I don't think you would be ruining it at all, you would be giving it new life. I can appreciate a restored truck, but they are of no use to me, as I put thousands of miles on a year and go on long trips with my vintage vehicles. If I have to load my vintage truck on a trailer to go to a show 11 hours away, I'm not going.
Everyone else can tell you whether or not you're 'ruining' the truck, but ultimately you own it, and you get to decide how the truck will best suit you.
