Eric, nice general description

This article implies 300mm lens limit

Extra bellows won’t help much if you can’t pull the standards further apart. 18” = 457.2mm

I had forgotten that I own the book, “The Linhof Camera Story”. It lists details on each camera made.
“Kardan Color 9x12 cm /4x5”
Camera Type: Monorail view camera, successor to Linhof Color
Made 1964 to 1975
Special features: Base rail splits into 330mm and 120mm sections (useful for wide angle shooting and for portability with rapid screw connection. Back has 70mm vertical shift. Cutout at front of housing for easier front rise with wide angle lenses. Rotating back with swing/tilt frame. Hinged rail clamp. Lateral shift (40mm each way) on lens standard plus center tilt. No lateral shift on rear. Maximum extension 435mm.”

Earlier in the book it refers to this model as the 20 year predecessor of the Technikardan. Greater bellows extension was seen as one of the improvements in the TK45.

I would guess then that 500mm would simply be too much for the rail limitations. The bellows would be cramped up and limit your movement. 435mm-450mm would be lots of movement for a 300mm lens.

Just to up the anti, if you want to shoot wide angle, you will probably also want a bag bellows. My TK45 had both by the time I was done.