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

Rare doesn't always equal desirability or an easy sell.
If you like it, then go ahead and buy it and don't think about resale.
I bet that there are a lot of slant 6 fans out there and would love to have this car especially because it's a 4 speed.
But, the price would have to be comparatively low to sell it versus a 273 4 speed car.
That would be a really fun car to cruise in, and good on gas to boot!