the monovalve would only affect cabin temp, not engine temp.
when it's warm out, no thermostat would result in a steady 80C temp, but it would take a LOOONG time to get there, when it's cold out, it takes a REALLLLLLLLLLLLLY long time to reach operating temp. best to get a good 175F (80C) stat in there. keep an eye on the temp from there, if it starts to get hot, you may have water pump issues, or a clogged up radiator.

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!