V8 to Slant Six swap: Sacrilege to must, but what would need to be done to have it happen?

You should state the year when asking. Looks like a 73+ based on avatar.
You are probably correct that you are more likely to find a 4 spd manual in a V-8 car. If 3 spd and 4 spd bell-housings are the same (I don't know), then easy to find a 3 spd slant car. If not, the bellhousing may be a tough and expensive find. A Tremec 6 spd manual would be even cooler, but doubt anyone makes an adapter plate for a slant.