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

As for keeping the slant six, back in the 90s I used to own a 65 Valiant 100, originally a 170/3-speed manual. Swapped in a 225 slant six and an early A body 4 speed with 3.09 low gear. Slant was pretty stock, but did have a four barrel carb, 220 @ .050 cam, head milled .100, and the stock exhaust manifold was hooked to a 2.25" exhaust system. One of the most fun drivers I've ever had. Not super fast, but I did outrun my brother's 64 Barracuda with 273 2-barrel motor slightly modified with four-barrel and dual exhaust. Also an early 289 V8 Mustang, stock Chevy 350 V8 pickups (back when they only had 170 hp) and assorted Grand Ams and Nissan 280Z's. With Addco front and rear sway bars and 14x6 slot mags, it also handled very well. Relatively speaking, it would be a lot slower today, but a Tork Storm supercharger could fix that.