I can only assume that the 34 lb thrust is the same force as a 34 lb weight would have on the transom if it were horizontal and the weight would be in the center.
You'd want to transom to be beefy enough - not only to support the weight but also not to flex.
Most of the transom failures that I've seen - that were not due to wood rotting underneath fiberglass sheathing - were failure where the sides join the transom.
I'd fall down on the side of a thicker transom - 3/4 or 1", not so much for the transom being strong enough to handle the motor but for giving you a lot of gluing surface for the sides and bottom
Just another thought -edge nailing into plywood isn't that strong so i'd go with coarse thread screws, as well as glue, maybe even fillet the inside joints
it's a lot easier to overdesign than repair....
