SM 465 stuck in 4th

[Adambomb]

Hey I have a 1970 gmc 4x4. I was driving to the city today and went to downshift at a light, and it wouldn't come out of 4th.. Its just stuck in there, the clutch is working still, but I reefed on the shifter and it just wont come out.. I still drove it home but, it was hell if I had to stop at a light and start in 4th.. oh well I got it home without a tow truck and luckily there was only 2 lights!!

I'm pretty sure its a sm 465 tranny.. I broke the roll pins for the shifter a few months ago and couldn't find the pieces of them. I ran a magnet inside the hole but came up with nothing, so I'm sure the pieces are in there still somewhere..

Any suggestions? Would a piece of roll pin be blocking it from shifting?
Also if I have to remove the tranny, do you think its worth getting a more modern one with better gearing? The engine block I run is out of a 2000 Camaro already.

Thanks for any help or suggestions!!
Adam