94 celica st overheating issue, 1994 celica st 1.8L overheating issue. Options

[rapid36]

Hi i have a 94 celica ST with the 1.8L motor. I bought a used motor from online that had 132k miles on it. i dropped it in and i replaced the following on the car:
1. rear main seal
2. oil pan gasket
3. torque converter
4. radiator swapped out with 3 row performance radiator meant for gt4.
5. added a 15 row transmission cooler
6. new air filter
7. new spark plugs
8. new spark plug wires
9. new front o2 sensor
10. new fuel filter
11. straight pipe headers
12. high flow cat
13. aftermarket muffler
14. new belts all the way around

and so when at idle everything seems fine and when i drive around town everything is fine but only when i get on the highway going 65MPH it starts to overheat very quickly and then i pull over and let it idle and it seems to go back down then i drive again and then at this point going any speed is making it overheat. i have no check engine light. and i checked the thermostat and it works and i checked the water pump and it works. i just flushed my cooling system and no luck there. I dont get why its overheating on me. I checked the compression and all cyclinders show 150. no noises are being made and when it gets hot the car runs fine no missing or other issues just gets hot. Anyone have any ideas?

[rapid36]

I timed it correctly with a timing light and the Haynes book calls for -10 degrees so that's what i timed it at. its funny how this car can never stay together i dropped the transmission like 3 times and the motor twice lol seems like it doesn't want to be put together. ill go by napa tomorrow and pick up that tester! I'll post back with the results. ill try to post in depth pictures of what im seeing so you all can see whats going on and hopefully after i figure this out that it will help others in the future. if anyone has any other ideas please let me know and ill go do tests or answer you back if i already did a certain test.

P.S. its really funny i never heard of the exhaust messing with the cooling system. you just taught me something today thank you! and out of curiosity since my compression showed good in all cylinders wouldn't the compression test show a weak cylinder if i had a bad head gasket?