I would not try it in the vehicle. I know the lifters will not come out without removing the heads. So doing it on an engine stand would be ideal... besides, those cams are ground on a smaller base circle so a pushrod change would in order. Otherwise the valve train geometry will be out of wack. I had Camcraft grind a custom cam for my 4.0 which required new pushrods. The 4.0 has goofy rocker arms that are un-adjustable and quite often wear out in the pushrod sockets. Not many options in the valvetrain.
Had I to do it over again I would have bought a new set of HD heads and had the ported along with the intake. I think that would have been a better bang for the buck.
I sympathize with OP not wanting to bother replacing the cam.
I have installed the COMP 410-8 Cam in my '95 4.0 OHV. So I can contribute a few notes for anyone reading this thread after the fact.
1. Worth it?
The 410 cam is pretty mild, and the stock computer can deal with it just fine. Without a tune or accompanying mods, it's hard to tell any difference in performance. If you're already that far into the motor for other things, have $300 to spend, and plan on future mods, it doesn't hurt.
2. Remove the engine?
I installed the cam while the engine was out for rebuild. As others have said, the lifters don't come out of a 4.0 OHV unless the heads come off. However the lifters don't need to come out completely to stab in a new bump-stick. If one is willing to remove radiator, intake, accessories, valve covers, it would technically be possible to leave the engine in the vehicle. It would be a COMPLETE pain in the ass however.
3. Pushrods?
Custom pushrods are not always required. Yes, with any new camshaft, measuring for pushrods should be done. However when I installed the 410 cam, I measured .019" preload on all the lifters once I installed new rockers and stock replacement pushrods.
4. New Heads?
If I had the money and was doing it all over again, I agree with evanesce69 and would have picked up a set of aftermarket HD cylinder heads. Yes it's polishing a turd, but I would like to have tried my hand at porting/polishing said turds to see what I can get out of the 4.slow.
5. Goofy valvetrain?
Yes. Yes it is. As much as I'd love to throw huge $$ after some roller rockers, the stock setup can be improved. I can confirm new Melling rocker arms have larger oil holes that feed the sockets and valve tips. Combined with a high-volume oil pump the top end can fair much better. If you want to make it as good as possible, get hardened pushrods and swap out the rock shaft springs with low friction spacers. Make sure the lifters have their full range of motion and are not seized/gummed up/plugged.