Fixed mountain climbing gear. Help please

50 inches is not that low a gear for a sustained climb. Generally for a climb that long I'd be using either 36x23 or 36x26 which is between 36 and 41 inches. I think you might gain some efficiencies from riding fixed but I'd still gear down if at all possible. That said, you'd probably get up on a gear of 50 inches but your knees might not thank you.

I assume you're going to do some tests before you commit?

I'm already commited (although I could take a different bike). I am trying to decide which cogs/chainring to buy, so I can start testing myself and the bike on more local climbs.

I would rather spin on the flats and post a bad time then not make it at all.

Hmmm 41:23 perhaps...

(BTW thanks for the gearing input)