My most recent 1957 Chevy has "One" change from completely original. It's the transmission from a powerglide to a 350. What will that do when I get the car judged? What class would I be in? How much will that count against the score?

IF the judges deteck it you will loose points

As a VCCA member you have access to the Judging manual. It gives guidance about points deducted for various flaws and discrepancies.

I’ve had 3 or 4 chevys judged by VCCA judges, none crawled under the cars to judge chassis etc that can be judged thoroughly laying on one’s back. Mike mccagh

There was a member of our judging team, at Bowling Green, who slid under each car to look at the bottom side. We were judging 1933 through 1939, so there was room to do this. I don't know what the teams were doing on the later and lower cars.
That being said, I find it unlikely that a judge would know the difference between automatic transmissions, whether they were installed or setting side by side on a table.
My advice is enter your car, keep quiet, accept the result with a smile.

Mike

From below at a quick look a 1957 with optional Turbo Glide could be mistaked for a 350 aas they both had removable pans and aluminum cases.The iron Power Glide had no pan.
More obivous would be the instruent panel shift quadrant if changed.