What you need is different gearing in the rears. The stock configuration in an 87 Wrangler, 4.2L, 5spd...is 3.07 ratio. I know this cause I own the exact same animal. lol.
Sure it would be nice to get a big 350 V8 under the hood, with an automatic trans. Would make a good combo for some rock crawling. But you will need to get a different rear without a doubt. The stock Dana 35 in the back WILL NOT handle the torque that the 350 SBC produces.
As far as that 5 on 5.5" bolt pattern...Do you already have different axles under it? Cause the stock bolt pattern is 5 on 4.5". The newer Jeep JK has switched over to the 5 on 5.5", but they are usually 17 or 18" wheels.
Back to the rear end...The common swap is a Ford 8.8, runs the same 5 on 4.5" bolt pattern on the axles, and if you get one from a 95 to 01 Explorer, you get disc brakes for the rear as well. Another good axle is a Dana 44 out of a Rubicon or some cherokees had them as an option. Or if you are for sure going to run the SBC, you might want to think about swapping out both of the axles for the ones from the suburban, but that will eliminate your 5 on 4.5" bolt pattern.