It will be tough to fault the new Griffin Radiator and that type of shroud. Top of the line parts there as long as you are running Spal fans and not some Chinese knock off. If they are not Spals, I would invest in the highest flow Spals available if you get no results from what's posted below.
Check and adjust your timing for max possible without hard starting or detonation. It will probably be in the 16-18 initial range with the vacuum advance disconnected. I see an HEI w/vacuum advance. Hook up your vacuum advance to manifold vacuum. If you can get us your initial timing with the vac disconnected, with the vac connected and your all in timing at 2800 we can offer suggestions. Running retarded will cause your engine to run hot as will running too lean.
Some guys will get by not running a bypass hose on a big block, If I were you I would try to run one. Temporarily move your fan switch to one of the other positions or into a head. Install a new thermostat 160- 180, I prefer 180 and it should work well with a functioning cooling system. EMP Stewart type is best all around. https://www.summitracing.com/parts/e...make/chevrolet
. But I would be happy if you just replaced what you have to eliminate that as an issue.
If that doesn't work I would throw a high flow pump at it. You may have an impeller issue. You may be able to feel it if you remove the belt and spin the pulley by hand.
All in all it looks like a first class set up. If it's holding water and you don't keep adding coolant, then your head gaskets are probably ok. If you have to keep adding coolant, I would run a radiator pressure test to rule out blown head gaskets. Keep an eye on your oil and the inside of you cap. Watch for rising oil level or gray watery deposits.
If you can supply your timing specs we can determine if that could be cause for your issues.