I have two 70s, A one owner LS-5/M-22 with C-60. Cooling is 100% stock, Down to the rubber/Stainless trim on the fenders near the windshield.
My driver is a 70 Nomad Wgn. with a 468 and a 700R4 and factory C-60 Converted to R134. Sitting in traffic on a hot day in the 90s with the A/C running the 468 will hit 215 at times.
Both of my 70s have the stock VO-1 cooling systems ( with ALL the details the factory installed) I run nothing special in the stock 4 row Copper Brass Rads. in both 70s. Green anti freeze from Advance auto and water from my garden hose.
A lot of cooling issue are Self Inflicted over the years with little changes here and there. I would start with a REAL FINE engine tune, Get the engine bay equipped as it left the factory. This may get you to " Square One ".
For over 50 years I have used nothing special to keep the engines KOOL. Spigot water and Green A-Freeze. On my driver 70 LOOK
Between the core support and the 4 row rad. You will see a space about 5/8
" to 3/4" GAP, I got some sticky back, black foam tape and closed this GAP.
Just get back to basics! You are NOT going to buy some magic LIQUID that is going to help. A lot of overheating issue are Missing parts under the hood. Chevrolet was not doing car buyers a favor when the installed Rubber seals, Splash guards, along with other items. If you do not have the factory style " DUCT WORK " under you hood with everything in place, This may cause problems???????
My one owner LS-5/ 4 speed did not get hot in 1970, It does not get hot today, ALL the factory parts & pieces are in place.
Just a little bit at a time and you will get it KOOL. A lot of folks on T-Chevelle are here to help. Just keep asking questions.