383 cam decision
I have a 67 belvedere with a 440. it is build almost to the T to what that 383 is. same cam and everything. with a ptc converter and 4.30 gears it has been 7.40's at 3550 lbs with me in it. I footbrake it.
I think there is a lot more than 7.70's in the combo you have. your car is lighter than mine and you have a trans brake (I'm assuming) along with more gear.
I know I have more cubic inches, but that 383 should still run close being in a lighter car. I kinda choke the 440 with a 750 dp carb as well.
I would rebuild as you have said with the thinner gaskets, and look at a Comp High energy grind. something around the same duration as the .509 cam you have but with a split duration.they are a little better grind than our old purple cams and are ground to take advantage of our .904 lifters.
also, maybe have the converter sent in to PTC and let them check and freshen it if need be. if the cam is changed very much the converter will need to be sent in anyway to be adjusted for the new cam specs.
WHOLE LOT of performance in a torque converter that is right for the combo. I was running 7.80's with a B&M holeshot converter, and went to 7.40's and 50's when I called PTC and had them build me a converter for my application.
good luck and keep us informed.