I pulled the 14' flatbed off the C600 I have and the frame has been extended two feet from 12' to 14' and done really poorly I might add. Is this a common thing to have done and how long can it be extended? 16' 18'?
Or should I keep it at 12' and cut off that 2 foot add on?

 

what are you planning to use the truck for?
If I were going to drive one, I'd make it as short as is proportionate, yet practical to my purpose. My plans are for a crew cab hot shot rig to pull a 5th wheel as shown in my concept sketch in my avatar.
shabby frame mods ought to be tended to nonetheless. late model big truck frames are heat treated. a mod on them involves no welding, only bolted connections with connecting pieces usually referred to as "fish plates". I'm pretty sure these old trucks are just oversized cold steel. if true, they could be appropriately welded on by a very well qualified welder. a frame can be stretched a very long ways. consider limosines. rule of thumb is a drive shaft carrier bearing at least every 60", most reccomend closer.
JML

 

upon second reading.... is the extension behind or before the rear axle?
JML

 

I am not going to extend the drift shaft just the frame part past the dirve shaft if anything.