OK, here's my take on spacers. It all depends on the rpm range in which the motor will operate and the cam you're using. If using a 180* dual plane manifold, you're asking the motor to operate from idle to about 5,000 and it will generally make good power in that range because any one cylinder only "sees" 1/2 of the total volume of the manifold and therefore can suck a vacuum and generate good velocity to fill the cylinder better than if you used a 360* single plane manifold. If operating in a range of maybe 2,000 to 6,500, the 180* manifold will choke the motor, so the 360* manifold might be a better choice to make power in the upper rpm range.
When you add an open spacer to a 180* manifold, you effectively turn the manifold into a 360* manifold, because now, any one cylinder "sees" the entire volume of the manifold just as it would if you started with a 360* manifold in the first place. Low rpm response will suffer while upper rpm power will pick up. You may think the car is faster overall because it picks up at higher rpm's, but I'll bet that if you had the car on a dyno, you'd see that the power under curve is lower at rpm's less than where the motor begins to breathe better. All this assumes a stock to near-stock converter. If you were to use a looser converter, then of course the motor would bypass the soggy lower rpm range and make more power under curve, resulting in a faster car.
I hope I'm explaining this well enough to understand. Let me put it this way: Let's call (no spacer) 180* power "X". If you add an open spacer, low rpm power will be down due to lower vacuum and resultantly lower velocity and less cylinder filling. Let's say at 2,000, you're down X minus 30 hp. At 3,000, you're down X minus 20 hp. At 4,000, you're down X minus 10 hp. At 5,000, it's a wash. At 6,000, your up X plus 10 hp. At 6,500, you're up 15 hp. It should be easy to see that you're applying more power under curve throughout the lower rpm range, more than could be made up for in the upper range. I realize this is not very scientific and is just a supposition on my part, but it all makes sense to me. I hope it makes sense to the rest of you.
I would welcome alternate points of view.