There both easy builds with a non ho you can vet a newer head to gain more power i dont know the difference i think there the same after the head but the only way to clean that engine is to tear down clean rebuild you could try valvoline high milage and just change every three thousand valvaline...
1) Wheel spacers are ok i run em but they have some issues when i first bought mine i threw em on proper torque started driving ten miles down the road i started to get a popping noise and a vibration pulled over and they were loose so you have to use lock tite but if you wanna deal with that...
I prefer 4 door cause i like the look more iys all in your own opinion but you know all xjs are unibody you cant just get a new body basically take axles and engine trans tcase you have a body you could get skins and doors tho
i dont have any problem with my welder at all and i know quiet a few that weld just fine along side any hobart or other welder plus if it breaks i can take it right down the road and they will replace it