Rebuilt engine, smokes bad at high RPM

Two questions.  Is the smoke black or blue.  black = unburtn fuel.  blue = oil.  Next question, did you alternate the groves in the three rings on the pistons when you put them in.  If they were all lined up then they will pull oil through until they get out of sync.  That is as long as they don't create a little grove in the piston wall during break in and then stay that way.  Also, did you file fit the ring gap?  Just a thought on the third question.

Jack

Modulator diaphram is good - it works fine.  I doubt it's a ring problem - I had it professionally rebuilt by a very reputable engine builder.

I had the car up on a rack last night and I think I may have found the source of the smoke.  There were a few drops of ATF on the tranny pan.  Well it just so happens that the night I had the smoke, I was running without the modulator valve connected (to vacuum), to see what the shift points of the govenor would be without any influence from the modulator.  Adam's Appel pointed out in another thread of mine that doing this is a bad idea, as it causes the tranny to produce to much line pressure.  Since I hadn't noticed any tranny leaks before this, I am guessing that the added pressure pushed some fluid to squirt past a seal in the tranny and hit the exhaust.  I now have the modulator reconnected, so if the problem goes away, I'll know that was the cause.

Chesh, that's a very good possibility, there. Fluid can be "slung" onto the exhaust, and smoke like that. It could happen whether trans fluid, engine oil, or even P/S fluid.
Usually when this happens, the oil will "burn" into the exhaust, and creat a dark spot/area on the pipe(s). This would confirm your suspicions.
Let's hope it's just a simple leak, and not a major engine issue!