I just checked your profile (thanks for filling it out BTW). If power is the issue, running 35" with 3.50 gearing is going to suck the life out of any motor. A viable alternative to an expensive and entailed engine swap is to do a gear swap. A set of 4.11's or 4.88's would Wake the 258 back up. Couple that with an MC carb upgrade, you hav e a very reliable , moderately powerful engine with minimum of effort.
That being said, there are a number of options. Do a search up in the CJ forum, there are a couple of build posts that revolve around SBC Chevy swaps.
I'm not a fan of the 305 and would never spend the cash on one, but for free it is as good a start as any.