The A518 will bolt right up to your 318, but you may have to come up with a custom mounting bracket and mount. Of course you'll need to have your driveshaft shortened. Just be sure that the A518 you use is rebuilt and the converter is new as well. The OD is electrically engaged so you will have to run a switch on the dash. My '94 B250 van has an A518 and there is an OD switch on my dash.
BTW, there is a Dodge Van section on this forum now so guys like us can hang out.
1994 B250 Ram Van Conversion by Elk
2008 Grand Caravan SXT
North Chesapeake Bay Vans, est. 1975