Maybe I'm wrong, but I think 115-118 for compression is way low. I just did a check on my bone stock 65 544 with B18 and got 155-160 on all cyl. The fact that the readings are even across the board suggests that something is affecting all cylinders.
Are you absolutely sure the cam is timed correctly? At TDC on #1 compression stroke, you should be able to slightly wiggle both rocker arms.
Also, the car idles a very long time in a super hot garage with no problem but overheats when driving. Try setting the timing without a timing light. At slow idle move the dizzy CCW and CW to achieve the fastest idle. Then just a very small amount CCW. This will be very close to or dead on the correct setting. This will eliminate any problem with timing light or slippage of crank pully.
Retarded timing will cause serious overheating and an incorrectly timed cam will do the same.
Good luck, Tom