I pulled my '37 gear box out from the bottom, the truck will need to be up on stands in the front. You should be able to wiggle it out, you might have to loosen the exhaust pipe. If it's original and never been out, its a good time to pull it and give it a good cleaning and adjustment. No need to replace it yet. Follow this manual, all the procedures are the same, https://chevy.oldcarmanualproject.com/booklets/52mts20/index.htm. Keep this link, theres all the information you will ever need, https://chevy.oldcarmanualproject.com