Newbie with transmission questions

[onepunchman]

Hey guys, I've been lurking on here for a while and learning all I can about my truck but I'm kinda stumped. I have a 69 short bed fleetside 350 2wd with a 4-speed. Most everything on it seems pretty original. Just a well used old truck. It was lowered when I got it but I like the stock look so I put the stock springs, stock height spindles and stock wheels and stock size tires back on it. Anyway I figured I'd need to change the speedo gear to correct my speedometer since it has bigger tires on it now so I looked at the tranny and was suprised after doing some research on here that it looks like it might be an sm420 not a sm465. The casting numbers on it are 3901127 and the date code is k208 which looks like 11/20/68? But everything I've read on here seems like they stopped making them in 67. Can someone set me straight on this cause I'm confused. Also did they put vin numbers on these so I could see if they match the truck vin? Another question I have is how do you remove the speedo gear since there's no retainer holding it like in other trannys but it looks like just a nut that unsrews to get it out? Thanks