Well if everything worked well before the rad swap and overheats after the swap, perhaps it's the new rad.

I agree with the others' comments about the timing. With the timing set so low the air fuel charge may be still burning as it heads out the exhaust port. That heats up the surrounding metal and the coolant. Maybe try 16-18 initial and try connecting the vacuum advance can to manifold vacuum port rather than the stepped port.

I once solved a friend's hot idle problem by simply changing the connection of the vacuum can from stepped to manifold. He had initial at 12 but the extra 20 degrees of vacuum advance cooled the idle temp.

Not to say it isn't one of the other suggestions, air can get trapped in the cooling system which would cause hotter coolant as well as thermostat etc.