Exhaust manifold temps on carbureted engines at idle shouldn't be trusted. The engine should be brought up to a resonable operating speed, preferably under load - then look at the temps. What do the spark plugs look like? If they're good I don't know if much credence should be put in the idle manifold temps.
I'm not real smart on FE's and don't want to insult you, but have all the baics been checked? Thermostat? Purge air out of block, heads, and intake manifold? Correct gaskets on the build? Using the radiator/fan shroud? Is the clutch fan pulling air at low RPM? Upper radiator hose temp versus lower radiator hose temp?
I'd like to talk to you about the engine - I'm going to build my CJ and I can use advice.