904 tranny problems

Backfiring when idling could be either a plug misfiring or the carb is running lean. Since it's sat a long time I'd tune it up and adjust the carb for best idle with the trans. in gear and the parking brake set, or someone sitting in the car holding the brakes on. The reason you adjust it in gear is sometimes if you adjust it in neutral then put it in gear it's a little lean. Putting it in gear loads it.

Trailbeast help me out on the shift shaft seal replacement. They make a tool so you don't have to drop the pan and valve body, don't they?