Most of the jumping out of gear problems were caused by worn out shift forks and worn out synchros. 48 to 53 used a 2 lug synchro. 54 up used a 4 lug synchro. A 4 speed will bolt right in and will probably be the cheapest solution to your problem. Depending on how much you care about looks, it used to be common to just cut a hole in the 3 speed floor panel.
