I put mine in the oil pan since there was a bung in it for just that reason(Dooley 14qt)
I might just mount the gauge for the cooler outlet somewhere on the engine where it is readable but out of the way so I don't have to cut another hole in the dash. What do y'all think?
You could also wire the gauge to be switchable between the two senders. Use a single-pole double-throw switch. Center pole to gauge. One sender to each end pole. Mount the switch under the dash. No holes to drill. Easy enough.