Removing TH400 tail section in the car??

[g356gear]

Hey guys,
I managed to pull the speedo cable and driven gear out of the side of the tranny last night and do a count of the drive gear teeth. I have an 18 drive gear and a 36 driven gear, which is the reason my speedo is so far out of whack. They were never changed when the tranny was swapped back in the early 70's to match the shorter 3.31 rear posi. From my calculations I have found I need a 47 tooth driven gear to match the 18 tooth drive gear, which nobody makes....max is a 45 tooth. This means I have to change out the drive gear inside the tail section of the tranny. Has anybody dropped a tail section off a TH400 while in the car or does the whole tranny need to come out? Thanks.