Timing by Vacuum Reading?

Here's how I do it without anything. Unplug the vacuum advance and block the hose if it has one "just because". If not, don't worry about it.

Next get it up to operating temp. Now, standing on the passenger's side, loosen the distributor just barely enough you can turn it.

Now, put one hand on the valve cover and get a feel for how the engine vibration feels. Next, start pulling in more (counter clockwise) initial timing. You'll notice the idle increase, that's good. That's the engine telling you it wants more initial timing. When you feel the engine start an extra "rumble" and kinda "beating" through the valve cover, you've gone too far, back it off until it just quits that. Now plug the vacuum can back in (if equipped) and go drive it.

Try to lug the engine in high gear at as low of a speed as possible. If it spark knocks (pings) back off a HAIR, like maybe 2 degrees and test drive it again. If if doesn't spark knock, then leave it alone. Nine times out of ten that procedure will work and be about dead on.

You'll still need to check with a timing light for the total advance and WHEN it is all in. You don't want a total of anymore than about 36 all in by about 2500. So, if you end up with like 40 or more, you're going to have to limit total timing. There are a couple ways to do it.

Don Gould at Four Seconds Flat sells the FBO limiter plate with different length slots that limits the total mechanical advance. OR you can weld the slots up in the distributor governor. I just did that recently to an electronic ignition distributor for my slant 6. I've done both. The limiter plate and welding the slots and I prefer welding the slots. The distributor has to come apart either way, so what the hell?

Lastly, you'll want to tailor the curve with lighter springs. I've done a LOT of them through the years and one light and one medium spring usually gets it RIGHT in the ballpark for most engines.

You can map out the whole curve if you want, but I usually don't mess with all that. For instance, my slant 6 has 23.5 degrees initial timing with a total of 33.5 degrees all in by 2700 RPM. It loves the crap out of it. It has a BIG solid flat tappet and lots of compression and runs like moody's goose. It's all in the tune. That's how I do it and it works for "ME".