Three to Four Speed Trans Swap Questions?

You didn't say what year your car is, but it has to be '70 or later since it's a Duster... and as such, it should already have the dual-pattern bellhousing. Three speeds never came with the oddball overdrive bellhousing with the oversize retainer bore, so you should be good there, too. EVERYTHING you have will bolt up, with the exception of the driveshaft. The three and '68 and later four speeds use different slip yokes (3 speed has a smaller diameter output shaft with fewer splines than the 4 speed).
Deal with that and you should be good to go. Swap the speedometer drive gear from the 3 to the 4 speed to keep your speedo accurate.