If you have a 4 speed it's more than likely an SM420. Search for SM420 on google, then see if you're looks like the ones you find pictures of.
I think you will find very few Chevy trucks like yours with tag axles and original 235, 216, or even 261 six cylinders.
These trucks could be loaded to more than the engine could comfortably handle with just one rear axle, add another and you still don't have any more power.
So if you load your truck to the capacity allowed by the tandem rear axle setup It should handle it but you will be going very slow.
Personally I can't see the need for a tag or tandem axle setup on a 2 ton Chevy; a larger GMC would have been a much better answer at the time, and still today.
Grigg
