I ran my RB locked out at 35*, and the motor loved it. It never hard started hot or exhibited detonation. I too wanted to add vacuum advance, and came across a relatively new distributor from Progressive Ignition that had a fully adjustable digital vacuum advance feature in terms of total added and at what vacuum. The rest of the distributor's timing adjustment was digital as well.

I now have the start timing set at 21, idle and total timing set at 35* and at high manifold vacuum it goes up to 42* with the addition of the vacuum advance.

I bought one and installed it last year. There is a small learning curve as far as set-up and dialing in, but it was quick and easy to learn, as I can see where the timing is in real time while driving the car around. Yes, I too follow the Four Seconds Flat method of hooking the advance to manifold vacuum.

Here's the link:

https://progressionignition.com/shop/ols/categories/chrysler