You need a vacuum advance distributor with vacuum source selected to manifold vacuum and not ported vacuum.
If not, your car will most likely overheat in low speed traffic and at idle.


I've had several people argue that they're doing fine using ported vacuum, but what most people don't realize is that GM developed the ported vacuum system so the engine would run hot at idle and low speeds thereby improving the nox and hydrocarbon factor.
Moh bettah for the smog situation in other words.

GM did have a thermostatic switch to swap over to manifold vacuum when the engine got hot enough and the theory was it would cool down.
True, it did, but sometimes the thermo switches let things get too hot and occasionally engines suffered heat damage.

And . . . some guys are bad about admitting things.
Friend of mine stuck a mechanical advance only distributor on the 327 engine in his roadster.

He claimed it didn't overheat in traffic, but later on I found that the reason he didn't want to go anywhere except Saturday night rod runs and car shows was that the engine overheated.