I'm agree that best option is the 4.3L but remember that the sporty is not does not have heavy weight components to receive all that power without hurts, in my case i had a diesel engine and components like R&P transmissions, etc are differents and works on a different ratio than the gas ones, 4.3 means change all drive train, 2.8 I maintained my Kia transmission, transfer, etc
It's not the same Modify an Sporty than buy a Blazer K5 and put the Sporty cabin on top.

Cost for my Swap was arround 2.5k including labor, suspention and parts.

4.3 would be more expensive for me.

When I was looking for an engine, one friend was selling me a 258 engine, reason why i didn't took it, because had to change transmission (t18), transfer (dana 20), cut the front of the car, more body lift, in order to re-locate the radiator and all those bigger components adding to much weight, incluseive change the diferentials to danas, front dif is an aluminum one IFS, not too heavy, rear one an handle more stress.

This is my daily drive and my super modify rig is a 79 CJ7.

Agree that next step will be take off the IFS and replace doth dif with danas 4.56 R&P actual ratio on the OEM ones that I have & mabe a biger engine, at this moment I'm planing to put them Buick aluminium heads, overhaul that engine with oversized pistons and instal it a Turbo...

After that will definetly fly, but it's a long term project