[maxwoof]

My experience with the older trucks is that the vacuum source is ported - the source is just above the butterfly in the carb. When you step on the gas, vacuum becomes present and the vacuum signal to the distributor advances the timing.

There are distributors out there that use vacuum from under the butterfly (constant at idle) to retard the timing at idle. In this case, when you step on the gas, vacuum drops, and the signal to the distributor allows the timing to advance.

In either case, the timing should advance as you accelerate. I would think that if you are not sure how your distributor should work, put a timing light on and observe the timing at idle. Apply vacuum to the distributor and see how timing is affected. If it advances, then it needs vacuum to increase off idle to advance timing as you accelerate. If the timing retards when you apply vacuum, you need vacuum to decrease off idle to advance timing as you accelerate.

In the first case, the distributor should have no vacuum at idle, in the second case it should. Hope this helps.
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