I believe it is easier and safer to have 2 pmds mounted next to each other. 1 oversized or 2 normal heat sinks based on preference. The one you currently use will have the extension cable plugged into it. The spare pmd electrical connections will get dirty,mud filled, etc. if left exposed. Buy a second connector piece for 5 or 10 dollars, and fill the backside with silicone- NOT the pin side. Let it cure on the bench a day, then plug it into the spare pmd.
When you need to swap pmd, all you do is take out the dummy plug and move the cable over to the spare pmd ready to go. Almost all of us have had a pmd go out, and it is never when you want it. In a hurry while raining was fun, that was the last time for no mounted spare for me. Next was during vacation on the freeway-it was a blast! Actually had the wife ask me on that one "Did you do anything or did it just fix itself?" because I hopped out, moved the wires over and we were on our way in 10 seconds.
Btw, I spoke to Leroy- He will sell the extra connector piece individual. Have a friend with a 96 hummer that I am helping fix currently. {@Burning oil - He is ordering ffm rebuild parts currently from you- spare pmd and such soon to follow. Apparently diesel should stay inside the ffm instead of all over the outside? haha.}