Good afternoon,If the picture in this query is the 383 you are having issues with,let me make some suggestions.

I have built several 383 engine here in FL. (VETTER BOB in this forum) and in CA. and the first thing to address and that is you will have to go with a mechanical fan,preferably a 7 blade which they are hard to come by,but can do with a 6 blade.It can be installed with your present pulley with adapter, I had the same pulley on my '80 Vette..After you install the mechanical fan,then we can work on my next suggestion.

Connect a vacuum gauge to the vacuum port in the intake manifold behind the carb or the non metered port on carb,however the intake manifold port is more accurate..Depending the gross lift of your cam the vacuum will be about 9-12 inches optimum of mercury again depending on cam lift.Get the starting vac.. and record the inches and advance the distributor to increase the vacuum if it will,if not set at between 9 -12 inches at the highest you can go without advancing to far and if it does increase vacuum then it will increase idle speed, reduce idle speed as vac. increases to set at 850-950 rpm.If you are advanced to much you will have resistance( bucking and "rumping))  in spinning engine..If all goes well you can install your timing light to get actual timing by gun..

Good luck and success..                          nosal          Sal C

Good morning .I found a picture of my Vette with a 7 blade mechanical fan.I later installed a fan clutch and electric fan which comes on at 182 degrees..I take no chances as Vettes are notorious for running hot.  & blade fan and extension..

Sal C

Thanks everyone for the suggestions. I am now able to get 35 degrees at 2500 rpm and 10 degrees advance at idle. A short test drive seemed to produce good results. My starter crapped out and I'm awaiting a new mini high torque 14:1 starter to give it a longer trial but I think I'm ok.

Was it the distributor advance springs?

Look into a advance timing light vs the digital one. The instructions are simple & you will get  the timing set perfectly especially on a modified engine.  Use some water wetter & use a 25% antifreeze 75% distled water setup.  If it still heats up you have other problems.