I was never able to find one for my '57 3600 after a long search a few years ago, so I improvised.
I'm assuming that you have an SM420 transmission in your truck with the floor parking brake attached to it the same as mine. I don't think that the Chevy came with a boot that goes above the floor, just one that goes below the floor...not sure about the GMC. I wanted one for above the floor to provide a finished look, but I also wanted to replace the one under the floor to reduce breeze from coming up through the floor.
For the one that mounts under the floor and is secured by the rectangular bracket that screws into the transmission floor pan, I modified a rubber shifter boot to make it fit. Here is a link to one like I used:
http://www.summitracing.com/parts/cwa-53004433/overview/It worked OK, not perfect.
For the one above the floor, I modified a different shifter boot and secured 'whiskers' to the inside of it using black silicon. The whiskers create a slot for the lever to travel through. The whiskers are the things that you see along the base of escalators that your legs rub up against. They are also used along the edges of commercial receiving doors at warehouses. I got a couple scraps from the maintenance shop at a local warehouse.
I am very happy with how this one came out. The rubber floor mat slips into the groove in the bottom of the boot to hold it in place. It is held in place by the groove, not glue, so it can be easily removed. I installed it about six years ago and it doesn't show any signs of wear and has never slipped out of place. Here is a picture--
http://s844.photobucket.com/user/ma...571_zpsfc372823.jpg.html?sort=6&o=49Here is a link to the boot that I used. It is kinda pricey to cut up, but it is the only alternative that I found that would accommodate the length of the brake lever travel.
http://www.summitracing.com/parts/bmm-80668/overview/Good luck finding a replacement. I will watch your post because I would also like to find out if they are available.
Matt
