Don't overlook the LSx engines in the modern trucks. LS6/4L60E can be had cheaply and is more or less the same engine as a LS1 except with a iron block. What difference does the 50 lbs weight difference make in a Jeep? Plus it'll still be lighter than the I6 it's replacing.
I'm not sure what the displacement on the LS6 is, likely either 5.3 or 5.7 but definitely an upgrade regardless.
Google should get you some info.