Did you use the '79 original motor/tranny mounts? If so, try to find a 'shaft from a short wheelbase halfton with the TH400 tranny.
Or, you could just take your trusty tapemeasure to the boneyard and spend some time crawling around and measuring. If you find a driveshaft out of a car that's the correct length but the u-joint yokes are of a different size, you should be able to obtain a "crossover" u-joint from your local parts store. Hope this helps.
Regards,
Wannabe
