Okay.
I don't think you'll break that with a 408 and auto (I'd look into a later model aluminum case 727, in place of your iron case trans, I wouldn't even bother looking for an adapter). But I'm also willing to bet that ....
1. There's not much gear in that rear, and
2. There's no suregrip in it.
So, one wheel peel for a couple hundred feet, with a b.p.
Personally.....
Imo, I'd buy the lesser version of the 408, and plan on using the savings for a aluminum case 727.
Better yet!... a 518 overdrive swap. (Find out what gear is in the 8 3/4.)
You do want to drive this, yes?
(I'd use the Iron two speed for keeping my trash can from flying down the street in the wind).
Yes, she’ll be a driver, but not daily. If replacing the trans, how would I know which one to choose? A quick search online generates numerous options, which vary widely in $$. Will the push buttons currently used for the power flite work with a 727 trans?