if you have a RB20 in a car not designed for it, your always going to have dramas
Example
The old RB30 in VL commys and R31 skyzerrs. In the R31 there wern't many heat related failures, but in the VL with its smaller air intake space between the headlights there we're many a cracked cylinder head due to overheating, mainly in the Turbo Calais. That was the in part due to GMH rushing to source a new engine for the VL after the death of the 202 Straight six back in the early 80's.
anyway....just go and get your cooling system pressure checked or whatever they do