I have been where your with your TC. I have a 1988, 4000 lb TC I had on my modified 2500 Dodge SRW for over twelve years. I recently purchased a 2004 Dodge 3500 DRW for its creature comforts.
There are many modifications that can be made to a given truck to improve its handling weight over its rated capacity. You will have to decide if you want to be absolutely under the manufactures rated capacity or would be comfortable making modifications to improve its handling weight over its rated capacity. The second issue is the fit of your older TC on newer trucks. My suggestion is to start two different threads.
One on the subject of load capacity and modifications to improve/change handling characteristics. You will need to know the weight of your TC, its length and if you intend to pull a trailer. Be prepared for a heated discussion. I also suggest you do a search on this Forum.
When you consider the fit of your older TC on newer trucks, the typical issues are:
1 location of the Tie Downs
2 Wiring harness. Where does the TC connect to the truck.
3 TC overhang height, compared to truck side rails (truck rails taper)
4 Bottom of TC to underside of TC cabover, compared to truck bed to top of truck
cab plus 2 inches (4" preferred, TC can be raised with mats or platform)
5 Widest width of TC below overhang compared to opening at tailgate and distance
between wheel wells.
There may be other considerations, but these are the most typical.
Wayne