Easiest thing to do is go to NAPA or Autozone/O'Reilly and order a Chevy 250 inline six unit. They are manufactured by Cardone Industries and are ready to drop in EXCEPT...you will require the dizzy gear from an AMC V-8. Best quality is the Mopar one, or you can try the Crown part.
Either way the replacement is very simple:
Push out the tension pin in the gear neck and remove gear...slide on the new gear and reinstall the pin...Ta da!
I have tried the HEI from the Camaro, Nova, and S-10 (early 70's up). All of them work. Each however is tuned a bit differently with weights and springs ...so you will want to tweak.
One thing you should do, even if you use a DUI...Measure the clearance between the installed gear and bottom of housing...you may need to add a hardened washer or two above the gear to get a final clearance of .010 to .012 ( the gear will walk upwards while running and change timing if you don't do that, even on Davis units). The shaft will fit a standard 12mm or 1/2' hole washer!
Cost??? About $110 for the dizzy and $35 -$70 for the right gear...about HALF of the DUI or Mallory...