Ford firewalls are almost exactly the same dimensions from 1967 through 1997. If you don't screw up the drivetrain angle and you leave yourself an inch fan blade to radiator clearance the only thing you will need to do with a 12V is massage the firewall a bit to clear the rearmost exhaust manifold bolts. The engine fits fine with no modification, but if you want to pull the manifold off in the truck you will need some room to get those bolts unscrewed.
If you have AC the 98-02 dodge manifold won't clear like it is. You can flip the manifold which will bring the turbo in several inches and massage the AC box about 1/2", you can buy a ford/freightliner/industrial exhaust manifold or you can port a late 24V/commonrail manifold to fit and turn it upside down or you can build a custom header. My advice is avoid the Ford/industrial manifold. They seam to hurt performance and are very prone to cracking apart.
If you are using a 24V the story is a bit different. They fit fine in 79 and older Fords, but in 1980 and newer trucks there isn't adequate room to remove the single valve cover without some massaging (quite a bit). 12V valve covers don't have to come straight up to come off so they clear fine.
The only other issue I've run into is with a 12 valve or 8V 4BT in a 67-79 Ford when changing the head in the truck the rearmost pushrod doesn't want to come out without hitting the flange where the cowl/firewall meet. I overcome this by holding the pushrod up and in place with a small clamp when pulling/installing heads in these trucks.
Autoworld is a joke. Mounts for 1980-97 fords are very simple. If you have a 460 or diesel truck the frame stands are the "big ones". Toss the diesel mounts, they are difficult to adapt, rather buy a pair of 460 rubber mounts, flip them upside down so they clear the Cummins block (the Cummins engines are wider than the 7.3 or 460) and build some real simple engine side brackets from 1/4" or 3/8" plate.
Also, do not try to center the engine in a Ford truck. They are offset for a reason. You may think you can gain more downpipe room if you center the engine, but for the headaches you will create it would've been far easier to get oval tube or route outside the frame rail.