Rarity (and desirability) of a slant six/ factory 4 speed?

By limited potential, I mean you'll never get a decent exhaust header on it unless you chop the car up.

That was true in the 70's, I bought some Kustom Headers for my 64 Barracuda. Cut for the passenger side, looked at tire clearance, and never cut the drivers side. Sold the headers and ran stock manifolds. Today you can get TTI's. They will even mod them for small exhaust ports on 318/273 and MAGNUM heads.