Get the PHS for sure, the headrest seats are from a '69. If you had the original engine/trans, then I'd say yes, rebuild and reinstall. You could do it now, but wouldn't be a numbers matching car anyway. It's probably worth more now than it would be with a non-matching 326 cube. Matching original motor, may be worth what it would bring on the market right now with that 400 in it. As far as rarity goes, yeah it is a one year only engine option but rarity and desirability are two different things...hens teeth are rare, nobody wants them. Hope this is clear as mud

Welcome aboard, lots of good info to be found here.