I have a 99 f250 with 8 foot bed and a GVWR of 4125 I would like to put a camper on it and was wondering if I could beef up the rear suspension with some heavy duty springs from a f350? Any advice or criticism greatfully accepted.

 

i dont think they would even fit.
totally different trucks and frames.

 

I think you are confusing with an F150

I was under the impression F250 and F350 are same frame, differences in springs and axles. I'm a little confused at you're statement that a 99 F250 is NOT a super duty, I thought they were all super dutys from 99 on?
Rear springs should be the same length. I put Super Duty springs on my '88 F250, I don't know the exact year, but they bolted up just fine. The fronts are a different length and take some modification to work.
Depending on how much you want to add weight wise, an 'add a leaf' kit may work. If you don't want to affect the ride with the camper out of the truck, air bags might work to remove the extra squat when the camper is in.

 

there were 2 F250's from 97 to 2004. the f250 light duty which was a F150 with 7 lug wheels, and the F250 Super Duty.
two completely different trucks.
and since he says it IS NOT a Super Duty F250, that means the only thing the same is the name.

 

Yupp its a light duty 7 lug. Long bed anything I could do to help the suspension (other than a new truck)?

 

Ah.... ok, That would be totally my mistake as I didn't realize that was such a difference.... oops

The other parts of my answer about an add a leaf kit, or air bags should last. You'd have to do some research if noone else answer. Google may help you.