Banjo & Josiah,
Well, what you fellas are tellin' me makes sense; the situation I "inherited" when I purchased the truck sounds like it was a case of a "ported vacuum (TCS) distributor" & advance hooked up to an older carb that furnished manifold vacuum to the brass distributor fitting.(Previous owner probably thought:"A vacuum line is a vacuum line.")
So, I think I've created some sort of "ported vacuum" by moving the location of that hole into the throttle body to the upper("atmosphere") side of the throttle plate; it mightn't be perfect (which is why I "operated" on the spare carb), but I'm gettin' closer!
Soon's I get a little "mad-money", I'm going to get a distributor & vacuum advance for the earliest 250 application I can find, and hook that up to a Rochester BV that I personally removed from a 230/250, and see how I'm doin'.
The way it was before my "elective surgery" to the throttle body, she'd spike to almost 25" of vacuum if I closed the throttle while going down the highway; that'd turn lots of things inside out!
