76 340 duster overheating at idle

FYI:
I run a stinking ancient, factory 318 rad circa 1973>75, in the factory slanty core support, with a 400plus hp 360 which will pull itself around all day at 550rpm and with 5 degrees of advance.
The rad in your pic will be fine, AFTER, you do the following;
1) install a shroud, and
2) Restrict the bypass hose,
3) move the clutch up closer to the rad, and
4) install a hi-flow pump, or at least an anti-cav. plate on the current one, and
Probably optional for you are;
5) If you have to, install a Thermostatic clutch or a direct-drive fan and
6) reduce the size of the w/p pully closer to the crankpully or slightly smaller than

Opinions;
1) A bigger rad, in your configuration, will NOT solve your problem.
2) After the system is functioning correctly, you can try a thermostatic clutch... but not a chance would I install a standard viscous-drive clutch, on an engine with a history of running hot..
3) as you have found out, measuring the hose temperature is meaningless.
4) IMO, This is an easy fix, and you will not need a bigger rad.

What you are actually looking for is a DIFFERENCE in temperature of close to 30 degrees, (minimum 20*) between the inlet side of the pump, and the outlet side of the stat house. Your system can't do it because;
1) not all the air that the fan is pulling, is coming thru the rad. Without the shroud, the air is just sneaking up from below, or simply recirculating the hot underhood air. The pump pulls whatever is easiest.
2) The pump is recirculating hot water from the bypass hose
3) the pump may be internally bypassing water around the open-sided vanes, and or, may only have a 6-blade impeller. and
4) if your pump-pully is BIGGER than the crank pulley, then it will be turning at fewer rpm than the crank. Most guys will tell you that this is a no-no. Whereas, I maintain that, with a pump that has the anti-cavitation 8 blade impeller, AND with a restricted bypass hose, I'd leave that change for last.

Other
>I would probably reroute that fuel line back to where it belongs, NOT running above the heater hoses and header. At idle, your header-pipes may run in the 400s, whereas the intake at 200 or so. Your IR gun cannot measure air-temp, lol.
>How close is your battery negative cable to getting into the fan?
> your KD spring looks a lil wonky to me.

Happy HotRodding