I have four parallel clamps made by Bessey that have a clamping capacity of 36" long. These clamps work great and I've owned them for about two and a half years now. The model that I have are the K Body Revo Jr. and have up to 900 lbs of clamping force. They come in 12", 18", 24", 36"(mine) and 50".

Now Bessey sells some larger ones called the K Body Revo but they cost much more. In fact, you might call these larger ones the big brother to mine because they have almost double the clamping force. But after buying the ones that I have I found out that Bessey sells an Extender Kit for the larger K Body Revo but not for the Jr.  These extender kits connect two clamps together almost doubling the clamping capacity which is a great add-on.

I've had two projects that I could've used this extender kit on but I ended up using some older style pipe clamps instead, which don't work very well. This brings me to my new project, making some clamp extensions for my K Body Revo Jr. clamps.

Metal Working

Here you can see two of the four clamps that I own. Now it's time to make some changes and some extensions.
 

Those two rectangle pieces setting against either other can be used to place under your work if you'd like and the one with the hole is to keep the moveable jaw from coming off the end accidentally. But this end piece is removable so you can take the jaw off.
 

My plan is to use the holes at the ends to fasten two of these clamps together with, creating one long clamp. But after measuring the holes I found out they are off center and vary where they're located. That means I'll need to machine the ends of the clamps and open up the holes to place them all on center.
 

With my 1 X 2 X 3 block clamped down at the rear, I was able to use one of the tapped holes along with a shoulder bolt to locate my Bessey clamp. Once my Bessey clamp was clamped down, I removed the shoulder bolt and then machined all the ends to the same length.
 

I'm using some leftover steel from my 33 Ford build which should work good for my clamp extensions. The thickness of the plate is 3/16" and the orange brake booster rod should work good for some bushings. Both are made of steel.