The 4.0, depending on the year, is rated 177 HP for 87-90 and 191 HP for 91 and up. It would be possible to use a donor 258 short block and put it beneath the 4.0 head but due to the smaller bore of the 258 the cylinder head flow would suffer and possibly a bit of performance too. Installing the 4.0 head on and existing 258 is a definate plus, but I'm not sure I would opt for putting a 258 short block in place of a 4.0. Now some people are taking the 258 crank and rods and installing them into the 4.0 block and pistons. This gives you a result of 276 cubic inches. Some tuning must be done such as bigger injectors and maybe an adjustable map. I have the jeep stroker in my race car and it runs very well. The details of the stock install of a stroker in a road vehicle are a bit unclear for me but I can direct you to some info.
