1976 chevy monte carlo 350cid 4bbl cal.

you will need to re stab the distributor, and re-set timing to factory spec. if you have slop in your timing chain your timing marks may move around quite a bit as you watch it ....if it is getting hot check to make sure your new intake gaskets did not block off coolant flow to the cylinder heads, if wrong or improperly installed they can cause your issue...

Just a note to add the harmonic balancer tends to slip around giving you a false reading. i have had 3 of them on older cars that have done that. you may need to replace timing chain, and sprockets as well as the harmonic balancer.
As far as the over heating goes, i can only guess that maybe it is either the thermostat or the manifold work you had done sorry i couldnt be any more help