My S2 Montecarlo I have just finished restoring also with my Fiat X19 Lido that I have posted a few times on the X19 forum...
I had the interior inc headlining rear bulkhead etc in leather and while I had the seats in bits I put in heater elements that are the 2 switches behind the gear lever also new carpet throughout...
I took the spare wheel out plus the air box and put in a K&N filter I had to move the water expansion tank to leave room for a small boot in the rear for my golf clubs...had no overheating issues at all.
Finally I altered the air filter so the car could draw colder air...
I had the interior inc headlining rear bulkhead etc in leather and while I had the seats in bits I put in heater elements that are the 2 switches behind the gear lever also new carpet throughout...
I took the spare wheel out plus the air box and put in a K&N filter I had to move the water expansion tank to leave room for a small boot in the rear for my golf clubs...had no overheating issues at all.
Finally I altered the air filter so the car could draw colder air...