I would keep the truck as stock as possible. No big deal IMO that the engine block is off. Being that it was purchased new by your dad and you have the original bill of sale makes the truck valuable especially sentimental.
My dad and I redid a 64 GMC LWB when I was a teenager growing up. It had a 283 in it that dad mated to a 3-speed overdrive. It was a nice truck and very 'snappy'
Wish I still had it. Periodically I search for one to make into a tribute truck. Usually what I find is junk or so expensive that I just can't see doing it.