Diesel in '66 3/4 ton?
 

I just bought a '66 GMC 2WD pickup, I've always wanted a cummins diesel p/u, but I got this instead. Would this be possible or wise? I'd like to keep it mostly original, but have some pulling power and fuel economy. I heard Fed-Ex was replacing their engines with a four cyl. Cummins. Anyone know about this? I'd like to replace the drivetrain, rear-end too, so that isn't an issue. Will this ruin it's resale value, should I decide to sell?
Thanks