A 3100 isn't much lighter than a 3600, both are going to weigh more than 3000 pounds but probably less than 4000. Remember that the weight of a modified truck depends on the modifications and may very quite a bit.
The IFS failures I've seen most press about have been Mustang II types, and the problems appear to me to fall into two catagories, First is modifications to the suspension design that are poorly engineered and rob the system of strength.
The second is poor craftsmanship. This kind of failure can affect frame clips, frame swaps and stock suspensions too. If the person doing the work doesn't know their stuff, and pay attention to their task, the result can be failure.
The Mustang II problems seem to relate to removing a strut that many feel doesn't look good. The result puts too much stress on parts not designed for it, sometimes resulting in failure.
As for concerns the truck might be too heavy for the suspensions you are considering, compare the truck's weight to the donor vehicle's weight. You may be surprised how much some of those cars weigh when compared to an old truck.
