My question is: Is your cooling system in your 70 a VO-1 or a copy of the VO-1? Why I am asking . I have a (ordered by me ) a 70 LS-5//M-22 with factory A/C.
My driver is a fake SS-468, 70 Nomad Wgn. with the VO-1 cooling and FACTORY A/C. My one owner has never overheated in the 49 years I have owned it. My driver with a 468 and a 700R has never overheated.
On a very hot day my driver with the 468 and the A/C operating will get up to 205-210 sitting in bumper to bumper traffic.
It runs at 185/ 190 on the highway with the A/C operating. Normal driving town/ highway it runs always around 170--185/190.
Also on the core support, LOOK
between the Rad. tanks and the core support, You will see a GAP about 5/8"-3/4" wide. When the car is static or moving slow, This large GAP will allow the SUPER HEATED air in the engine bay to be DRAWN through this GAP and back into the Rad. This will cause the water temps to go up. I filled this GAP with some foam tape with a sticky back side from LOWES.
For a stock cooling system to be effective, It MUST have ALL the factory items in place. Both of my 70s have every tidbit in place as it left the factory and I have no issues with my stock 454 or the 468.
All of the factory item work together in concert, With factory items missing ,,, YES you will have cooling issues.