Gurks, If it isn't doing it often then I'd suggest waiting until it becomes more frequent and then take it to a transmission place and have them experience the issue. I think the job of converting to a 6 speed would be a lot more work than if you just got a built 4 speed. If you do get the box rebuilt then take it to someone reputable and they will make it plenty strong enough. Cost can vary with what you want done to the box, standard rebuild will start from $1800 and up to 3.5k with the works, high stall and cooler etc. A quick search in the search field for "auto rebuild" will give you plenty of threads on the old BTR. Example below. Regards, Benny.