Paul, I found out the hardway that those thru hulls were not the same size. I bought two of the same size, and I had planned to replace both, bit it did not fit.

   We converted our former C36MkI from CNG to propane a couple of years ago (using a modified locker that I bought from a member who was parting out his hurricane-wrecked C36Mk1.5).  We only did so because it became virtually impossible to get CNG refills; it was no small job to do this, as we had to replace a perfectly-working (albeit old) stove, run new gas lines, install a propane leak detector, a remote solenoid switch, and -- most difficultly -- figure out how to put a propane locker into the lazarette.
   I would definitely suggest venting out the transom.   Remember, the vent line has to slope DOWNWARD from the bottom of the propane locker at all points of sail; given the dimensions, it was hard enough to accomplish this venting out the stern. I can't see how one would achieve that outcome if it was set up to vent out one of the sides.
   We did not quite achieve all the ABYC guidelines (i.e., the locker door can't be reached without opening the lazarette), but I think we came pretty close.  
   I would definitely think that T-ing the vent line into a scupper line or other drain hose would not meet the guidelines -- the vent line should have its own through-hull so that any leaks can only travel one way (and not back up into the boat).  Installing a small mushroom through-hull in the transom is the least of the difficulties of this conversion!
   Here's a link to a good article on safe propane installations (and what NOT to do) by a surveyor: www.pcmarinesurveys.com/Safe%20Boat%20propane%20Installation.htm