Two-inch will not interfere with the fire-wall.
Three-inch will cause the IDI to come against the fire-wall and vibrate deafeningly.
I put a three-inch on mine when the IDI was still in it.
In less than a week, I took out the risers and sawed them down to two-inch.
I have never had any cooling issues, towing HEAVY, with either engine.
If I had it to do over, I would have added the body-lift the first week I got the truck; a body-lift makes everything else you ever have to do so much easier.
One project that I intend to do is to, one-by-one, cut off the body-mount brackets from the frame-rails and make them THREE-inches taller, with ability to adjust the holes between three-inch and two-inch, then bolt them back on, thus eliminating the spacers and getting the truck back up to three-inch lift; the Cummins has no clearance issues.
One
VERY IMPORTANT issue that many fail to address is
ORDER A BORGESON SHAFT BEFORE YOU START A BODY-LIFT.
The stock steering-shaft comes
DANGEROUSLY close to un-splining itself with even a two-inch lift.
I marked mine before and after, then removed it to examine, and
less than 1/4" was still in the sleeve.
The factory FORD shaft is a death-trap at best anyway, especially so after the rag-joint has been oil-soaked for twenty-plus years.