If you want to keep it all Mopar then consider one of the Dakota 5-speed trans. AX5 is a light duty trans but adequate for a flathead, the AX15 is bigger/stouter (suitable for v-8), there are others going back into the 80's. If you want to keep the stock bellhousing (with the rear mounts) then look for a trans with the same (or longer) input shaft length. And yes, you would have some other issues to contend with. You would need new rear axle although in the scheme of things the axle is the easy part. .