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

I'm still keen on getting a manual, either 3 or 4 speed, for my duster but I'm pretty sure that buying another car of comparable condition that already has a manual in it is as cheap or cheaper than gathering all the parts needed to do the swap.

What happens if I find a v8 duster with a manual and I want to swap it for my six cylinder? What would I need to massage or swap to get it to fit. Everybody always talks about doing for 6 to 8 but I've not heard much about 8 to 6.