Mild 340 8 3/4 gear and possibly converter question

It kinda depends on the case number that you wanna put gears in. Very limited choice of new gears for a 741. Better choices for a 742 or 489 case.
If you can find a case with 3.23s in it I'd run them. If you wanna put new gears in your case, I'd go 3.55s I'm not sure if 3.23s are available new any more.
(And 5.13s ARE ridiculous. I had 5.14s in a 90% race car with 32" tall tires, and it was too much. It ran just as hard with 4.56s)
If you go with the 3.55s, and decide you still want less gear, you can add a couple inches of tire height, and it will feel like 3.23.