If you really want 800 hp you need to start with an ls bassed motor yes you can get 800 from an lt4 but it is gonna be expensive and youll be better off with the ls headflow geometry which is something the lt engines can't compare to

 

I personally would not go with an LT4 due to how new and expensive the parts for them are for now.
It really depends how strong you want to build it,
If you want a good reliable trail rig with that much power, this is the combo i would go with.
i would just go with a gen 4 6.0 for the stronger rotating assembly and add a LSA supercharger, This setup will only run about 3000-4000 if you can find some deals and make good power.
For the trans, id run a 4L80E because they are strong, inexpensive and easy to get a hold of.
for the transfer case you cant be an Atlas twin stick for adaptability, strength, crawler gears and the ability to unlock the rear and just use the front axle to pull around obstacles,
then for the differentials with rock crawling id run a high pinion D44 up front and a 9in in the rear.

It wont be the most expensive setup, but it will be fairly stout and powerful.

I would not put a very expensive engine in any crawler because if you ever flop it or get it rubber side up and dont get it shut down in time you just wasted all of that cash. With the 6.0/LSA combo you could just find another long block for cheap and keep on going.

 

Wait wait wait, which LT4 are you talking about?

The earlier one based off the gen II small block (only in the 96-97 Corvette Grand Sport), or the newer 2015+ Corvette & CTS-V gen V? Two COMPLETELY different animals, confusingly have the same LT4 title...

I'm going to assuming you're referring to the newer LT4 because the older gen ones are pretty much impossible to find complete, and let's face it why would you want to... and would take so many modifications and expensive bolt ons to make 400+ HP... But I could not see investing the price of a newer LT4 into any S-chassis unless you're turning it into a trailer queen track only racer or a massive money pit of a show vehicle.

I'm going to have to agree with ChuckNTruck on this one. A mild built 5.3 or 6.0 will easily give you a reliable 500+ HP and cost a FRACTION of what an LT4 pull-out would

 

I would run a 14-bolt Chevy rear axle. They're plentiful, cheap, and almost as easy to set up as the Ford 9". If you're worried about ground clearance, you can shave the bottom and still be stronger than a stock 9".

 

I agree with that. But if going with a 14 bolt rear you might as well run a D60 up front. I have seen 2 full size blazers running this setup that i used to wheel with and they never worried about breaking anything in either differentials, both carried spare driveshafts and u-joints though haha. But much easier to swap driveshafts on the trail rather than axles.
I also say that because ive snapped a D44 axle on my old jeep with a 6 cyl and it took a nice long trailer ride home.

 

I totally agree. The only reason not to run one would be cost, but you can buy ball joint Super Duty axles for a pretty reasonable price. My front axle is a Chevy D60, and I probably have almost as much money in it as I have in the rest of the truck.