Congrats on your 5'er!!!
I have not done what you are asking about extending the gooseneck on the camper. But may be able to help give you some things to think through
How much length did do you need to add? 2 or 3 inches may not be a concern however 10 to 12 inches may be an issue of bending the frame by the longer goose neck. Your welder may have to add some reinforcement on the longer gooseneck to not deflect the frame. The heads up is, point it out to him so he can work through it.
The 3500 truck sits up pretty high, any idea how far up hill the the front of the camper will be once you do connect to the ball?
I'm some how guessing you might have a turning problem. Will your truck hit the camper when you are turning? With the truck being so much higher then towing the camper level the bed rail clearance would need to be checked with this angle in mind. And then there is the ball is 10" more forward. Will the truck bumper hit the camper?
Did your Granddad by chance raise the axle? This would at least help with the towing level problem. Some of these little 5'ers where set up to work on a S10 pickup...
A thought just came to mind as you do have a 3500. The brochure pin weight of the 84 F1850 was approx 570#
If you install a 2 1/2" receiver and buy a rise draw bar that can handle 1,000 to 1,200# dead weight, maybe tow it on the ball verses the bed?? This might actually put the goose neck in the right spot to tow level and you have no modification to the camper and gain a good capacity truck receiver. Some day you may want to put the camper on a smaller truck and then do not have to worry about the goose neck mod you did. Need to check what the dimensions this will come out as. Reese does make a 6" 2 1/2" rise shank. If you know the ball height of the camper when level, top inside coupler to the ground, I might be able to help figure out if this is even possible.
You will have no sway controls like towing a camper on the ball this is now a bumper pull but your sitting on 23% dry pin weight that may end up being 25 to 28% loaded pin weight which means it will tow stable anyway. Use the mass of your truck in your favor. I know this may look a little strange, but maybe not really any stranger then towing on the ball in the bed...
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club