There are 2 adapters, OEM used on Dodge vans and one from Firm Feel. Easiest way is to use the adapter from Firm Feel. It lets you use the power shaft coupler where the OEM van adapter reqires the use of a manual shaft coupler.
Cheapest way is weld in a 3" extension as posted above.
Also check the worm shaft size. Make sure the pitman arm off your power box will change over to the manual box you have, if not, you'll need a '73-up Abody manual steering pitman arm.