Good Luck, Ron!

I'm surprised Dennis didn't suggest you ash-can the center drain. Most installers don't like them in A/Gs, and most owners don't have them. You're getting a new liner--why put a hole in the very bottom?

I'd much rather see you put in a low drain in the side wall. That is FAR easier to get to if there's a problem and winterizing is a snap. The performance difference won't be noticeable, but the advantages will be.