DPX drive
 

Hello
I understand you may be familiar with DPX drives. I'm trying to buy one off someone and my impression is there are three and I choose which one I'm buying. He lists the following:
"These are brand new Volvo Penta DPX, DPX-R, DPX-S1 Duo Prop Outdrives for 8.1 or diesel OLD STYLE. They are 1.68 # 872989. 1.78 #'s are 3868639/ 3868642. And BRAND NEW TRANSOM ASSY #3868299/3869145"
This would be replacing an Alpha Gen2 in my 18' Donzi with a 350/300 Mag MPI. I assume I need to buy a bellhousing as well. Can you advise me on the differences between the drives and which one would be preferable for my application? I bid on this impulsively on Ebay hoping I'm buying what I think I'm buying. Any feedback and info would be truly appreciated.
T.M. Hayes