If its running good, just take the oil pan off and timing cover. Do all those gaskets, it should run fine. My 4.3 went 260k before i dropped the V8 in.
The heater core you need to take the entire dash out to do. Its a PITA but if that and an oil leak are the worst of your problems, i would not sweat it. While you have the dash apart might as well throw in a new blend door motor as well. A lot of times those go and the same process is required to get to it. Im currently driving one of these crew cabs with over 300k on it. While its not the original motor or rear axle, its the same frame and most of the body and all the other little bits. These trucks will last quite a while and do fairly well.
Knowledge has no value unless its shared with others.
Exploration Machine- 02' CC V8 swap. 300K+
Portable Roller Coaster- 07' MKV VW GTI
Its another Jeep- 98 TJ