Took me a minute to catch on???..you edited your first post.
I have no FHE on what you're doing, but my $.02:
A car regulator should work fine, my only caution is that they may be a higher flow capacity than what is necessary for a Monster, so it may act a little funny. They may also be physically large and thus tough to find a mounting spot.
IIRC, my S4 runs at 45psi. I don't have my service manual in hand, I'll try to remember to check tonight.