[scuderia]

Originally Posted by 66Submarine

That's a "low hump" trans tunnel; you can see in the second picture that the floor is different on trucks with a 4 speed SM420.

(I actually remember someone on here that had a truck with an SM420 or SM465 (later version that you have) that was installed in a low hump truck somehow, but don't remember the thread off the top of my head.)

Here's a page on someone that did the conversion: http://6066gmcguy.com/3-to-4-speed.html

Thanks for the link!