Unless you're deleting the 2-3 accumulator circuit, you'll need the sealing ring on that piston...Otherwise, you'll have a massive leak in the high gear apply circuit and direct clutch will burn. One should have come in the overhaul kit; they usually come packaged as part of a dedicated sealing ring kit which is included in the broader kit...
Cobra Transmission Parts sells them individually if one did not come in your kit. Let me know if you cannot find one and I'll see what I can do...
Yep, especially the trans shop owners that either don't know any better or are raking it in on parts mark-up (or for on-line shills like you are starting to sound like)....You know nothing about what you're talking about but by all means, keep talking...You're definitely good for laughs, that's for sure.