As an owner of both a 400 and OHC, both versions are great projects. I chose to save my base OHC coupe from the fiat most cammers fall pray to, the V8 swap. The OHC-6 is a great engine, and if you care to add some upgrades they can be as powerful as some V8's. They aren't cheep to rebuild, but neither is the Pontiac V8. Parts for the OHC are out there, and relativity inexpensive because most are pulled for the V8 swap.
If I were to take on another project 'bird, I'd op for another cammer, just because of the price of fuel these days. I get 16 to 20 MPG out of my humble 1 bbl 3spd manual base '69 'bird. But having a V8 gives one more of a grin when stepping on the gas. I'd have to side with keeping it original. But it's yours and your son's prerogative to do what you want. And that's what it's all about.
Welcome aboard Colorado! keep us clued in on the progress of your father/son Firebird restoration.
