The block was strenghtened twice since the 258, the last time in 96, not only making it stronger, but quieter and less vibration. Oops, that was 2, and I didn't mention the newer ignition systems from 99 up.
The only reason I said to use the nissan trans and transfer case was to make it simple to use the nissan engine, I don't have any experience with nissan transfer cases, so don't know much about them. I had 2 620s with 4X4 conversions, and both only used the 30 front axle and 20 transfer case, and had no problems. both were purchased new that way. The 79 had a detroit locker in the nissan rear, and a truetrac in the front, and was just about unstoppable.
Along with loosing the heater, you will lose room for your right foot, and the passengers left foot, but maybe you are different, and don't need feet! LOL!
Both of the "Jeep" engines you mention take more room than a Chevy, and are unreliable, just ask me, I owned a 74 CJ5 Renagade with a 304, that I traded for my 76 620 with the 4X4 conversion, because I got tired of fixing it every time I wanted to drive it, and only 2 years old! Never had any problems with either of my 620 4X4s, and owned the first for 3 year, and the 79 for 4.
I don't have anything against Jeeps, I own 2 Cherokees now, both 4.0s, and they are reliable and perform great, but in my experience, if you want a 620 4X4, go with a KA for power, it's all you need, and you don't need to butcher the firewall, keep it simple!