[mr48chev]

Since it is pretty well impossible to just go to the local wrecking yards and find a later rear axle that fits and has the right gear ratio and shipping one from the states isn't financially feasible in most cases
I'm not finding any comparable size European or Japanese or ? trucks that may be available to you that have 8 lug wheels. That would be the simple solution but it doesn't look simple. With six lug you would have a number of options.

With an automatic you may need 3.5 gears or even 3.8 to have any real road speed.

If you haven't bought the new wheels yet I would seriously look into changing the rear axle and changing the front hubs to match. 6 lug bolt pattern would give you a number of locally available truck axles to pick from in Toyota and Isuzu among others and maybe some we don't get here. You could get a set of six lug Bendix brakes from someone here or order a disk brake kit. that may mean that you can get most of the pieces you needed from the US and possibly locate the wheels you need there rather than paying extra to have them shipped and end up with a one box shipment for front brake parts and possibly spindles.