You're absolutely right 6PakBee. Even though the brakes work great, the hissing of air into the back of the booster needs to be corrected. It's acting like a small, controlled vacuum leak on the engine.

The push rod gap was .030 prior to installation, but the gap AFTER installation is what matters. When I push the pedal in ~1/8", the hissing goes away which tells me my pushrod gap is too large.

When installing the booster with the 4 nuts on the cabin side, I partially compressed the new white foam gasket. I was careful not to over tighten the nuts, but maybe they need to be a little tighter. That would draw the booster back to the firewall. If I draw the booster back another 1/16" (compressing the gasket a little more), it should close the pushrod gap by the same amount. A 1/16" at the linkage would translate to more than 1/8" at the pedal.

When it stops hissing I'll consider it good. The pedal feel and travel is good as it is, but if the brakes start actuating a little higher in the pedal travel that'd be even better.
If tightening the nuts isn't enough, I'll pull the master and adjust the rod.
I'll let you know what happens.

Seriously, these brakes feel like hydroboost. They're really sensitive like the new Darts that I've test driven recently.