Re: 4.2 -> 4.0 swap questions
Your questions are not strange but all you need to do is look around a little bit to answer them for your self. This swap has been done hundreds of times and discussed more than that. Just 20 or so posts back was the same subject. Use the 'Search' facility for the value '4.0' or '4.0 swap' and you should find more information than you will want.
To answer your questions specifically,
1, You do not need an adapter but you will need the crank position sensor from Hesco.
2, Do not know the answer to this.
3, Yes, but you will have to move the mounts from the old 258 block to the 4.0 block. One hole on the drivers side has to be enlarged to 12mm.
4. Salvage yards or find someone who is upgrading to a V8. There are many who really diss the I6 and they are suckers for selling it cheap. You need to buy it complete including starter, alt, computer, PS pump, fan blade, etc. If you buy one from an XJ, you will have to install a YJ water pump and fan blade.
5, I believe you should stick to the 91 HO and up. Some believe their are even improvements after that but I do not know what they would be. Go for low mileage engines, they are out there. I think it is too much work for the older style FI.
6. I do not think it has to do with Jeep experience as much as overall mechanical skils, tools, workplace, friends, self confidence, etc. It is not a one week end event for most of us. Some have done it in a week end. I used it as an opportunity to basically rebuild/replace many things while I was at it. I took about 4 months but went clear down to the frame. Working on my Jeep is much of the reward of owning it. I wouldn't have it any other way. But my Jeep is not a daily driver, it is all hobby.
83 CJ8, 4.0 HO/AW4, 2.5 Superlift, 1" Daystar body, 33x12.50 GY MTs, 4.10, Lockers front & rear, on-board air, Durabak cab and bed, Trailquest tank, lots of small stuff.