Figured I would add my .02 to this thread since I have had cooling issues. I will second Cal's experience with Mishimoto and the passages getting clogged. The first Mishimoto was installed with the original 4.0. I then built a 4.6 stroker (Russ Pottenger kit) with a very mild build , almost identical to Mark Wolfe's. I flushed the block multiple times after the break in procedure. Each time, I would see rust particles or iron on the radiator's magnetic drain plug. I then did the citric acid flush thinking that everything needed to be cleaned out.
Still had cooling issues. Jeep was still running hot. I was already bypassing the radiator for the transmission cooling lines and running a small B&M cooler. I decided to upgrade the B&M to a larger unit. The transmission cooler is installed in front of the AC condensor on the mechanical fan side.
Still running hot. I took an infrared gun and measured temps across the radiator and found that the bottom was 30 degrees cooler than the top. I suspected the radiator was clogged from the previous engine or prior to the citric flush. I also had a stripped overflow tube fitting in the Mishimoto and fortunately they warrantied the radiator. It was much appreciated, because as soon as I installed it, my cooling issues were solved and the temps across the radiator were within 10 degrees from top to bottom.
So I thought. During really warm days (110 ambient temps) with the AC running I was running 230 degrees at idle. Mopar water pump, Mopar coolant temp sensor, Napa standard XJ fan clutch, 195 thermostat, and TYC electric fan from Amazon, and holes cut in my hood to exhaust hot air.
I bought a 10" SPAL fan from summit and made a custom aluminum fan shroud. I suspected the TYC was junk and I was right. The Spal fan pulled way more CFM than the TYC and on those hot days, I would creep up to 220 at idle and never really further.
My Jeep with camping gear, tools, etc weighs 5200lbs verified on a CAT scale. Going up the 168 from Fresno to Shaver lake in 105 degree weather, I was running 55mph and hit 232 degrees with the AC off. That was the best I could get.
Recently, I drove from San Diego to Kanab Utah, with the same amount of weight and now have cooling issues yet again. With 80 degree ambient temps, I was hitting 245 degrees going up Baker grade. I suspected a bad fan clutch so when I got to Kanab, I replaced the fan clutch, flushed the cooling system and dropped in a 180 degree thermostat. Once again, there was rust particles on the drain plug. Once again, the infared temp reading was 30 degrees cooler at the bottom of the radiator compared to the top.
At this point, I'm kicking myself for not installing a coolant filter. And now, I'm on a quest for yet another radiator.