Changing 4cy jeep wrangler to a 6 cyl?

I own a 1990 jeep wrangler yj with a 4cyl 2.5 engine and ax/5 transmission. I would like to replace the 4cyl with a 6cyl and would like to know the best way to go about this? Is there a 6cyl that fits in better then the others? Would I also need to change the trans and all the wireing? Or can I keep everything else the same Thanks