I'm building a 6'9" ML/F spinning rod for stream smallmouth, 40mm reel spool and braid. I used the 27x and tried to get away with a 16 Minima Match guide as the stripper. The stripper guide currently sits 29" from the reel face. If I palce a size 20 behind it, it falls 18" from the spool face. Is test casting even worth trying without the 20 guide?

Sure,
Try a 16, a 20, a 25, and a 30 with all of the different lines that you might be likely to use on the rod.

You will not know if one is better than the other without testing.

Also, not only test the cast, but also test the retrieve of the rod and reel as well. 'Do you testing for both casting and retrieving with the amount of weight that your lure or terminal tackle is likely to weigh.

About 90% of the time for nearly all of my light to medium spinning setups, I will set the first guide at 20 inches and it generally works very well.

Be safe
Roger

One thing I have found when using braid on a spinning reel, you can usually go down in size for the first guide. You might have to play with the position of the guide to find the best spot for how the line comes off the spool.