For the easiest option look up ACE engineering if you want to keep the rover trans. Personally I'd get an a RW adapter (https://rw-eng.com/products/adapter) for the t-case and run a 6L80 or 6L90 behind the LS. They might even make a box to fool the rover ecus like the ACE kit. However I'd dump the HP slush box in a heartbeat if going through the hassle of a swap. Another benefit of the 6L80 is when off-road the first 2 gears are almost like having a crawlbox the ratios are so low. Buddy swapped a 6.0 and 6L90 into his LWB classic with the rw engine mounts and adapter and it's incredible (obviously easier with less electronics to deal with).

 

I have a 4.6 and a spare 4.6 block. IF I go down this road I will probably build back stock with a cam maybe. Of coarse do the whole top hat thing. I don't have a issue with the power at stock, but I also am not pushing it up mountains in Colorado.
I recently watched the video by TFL on the guy with a LS GEN IV and 6L80 swap. Looks pretty interesting due to the transmission being a 6 speed and having a 4.02 first gear. If my math is correct that might mean in low range you have a serious crawler gear and a better overdrive in sixth at .66. Best of both worlds. Keeps the LT230 which I think is really cool.

 

I avoid the TFL pretty much at all costs lol but I might have to watch that one. Yea my buddy's classic he went to a 1.0 t-case bc things were too low, 3rd gear for him in low is like our 1st in low haha. Plus that 6 speed has the ability to have tap shifting which would be pretty epic. I'd still throw a cam in there, it's super simple and new lifters and it will be a happier motor over all. Both stock cams I took out of my 4.0s were worn pretty good..

 

I hear ya on TFL. Link to the video below. Dude is out in Colorado. He has tap shifting installed. RW has a ECU solution but I don't know much about it in terms of install complexity.

 

I don't either, I just saw it for the first time the other day. I don't think it was an option when I looked at their adapters a year or so ago. I haven't seen his disco around, there are a ton here and we have a pretty active rover group in the denver/boulder area. I'm waiting for Roadside Werx to swap a D2 so I can go check it out. I'm in there for parts seemingly weekly or to just hang out and it's a never ending stream of LS or cummins swaps into defenders, d1s and classics. The swapped classic friend is the lead tech there. Also if you ever need any parts new or used give them a call, they have just about anything/everything stashed somewhere lol.

 

I noticed the exact same symptoms on my DII about 3 months after buying it. this was January and I bought it in november. I think the 12 hour drive home is what blew it. drove it 3,000 miles like that then went ahead and did the heads before summer in early May.I mostly did around town plus commute, and one 300 mile trip like that. I lost maybe 1/2" of coolant in the reservoir over those 4-6 months from when I first noticed it. It's an extremely common place for them to leak. I never once got over 183 degrees during that time and I never noticed any burnt coolant smell or drips. ambient temps were mostly 30-75 degrees during that time.

Run it and monitor it and look/listen for any new symptoms until you can get to it. lots of folks here can help you with advice to tackle it if you do it yourself, but it shouldn't take you more than a a week of downtime. remove the heads on a Saturday, have a machine shop surface them during the week and do anything else it might need, and then reinstall the next weekend.