Most guys say it's due to lack of wheel selection. Not saying that is the case here.
The justification is usually all the wheels available in that pattern, never mind most are not the correct offset.
I don't get it, there are some sick 6-lug wheels out there. You just have to look a bit harder.
To answer the OPs question, if you want to do this correctly.
Pull the hubs and axle shafts, take them to a machine shop and have them re-drilled for the pattern you want.
Also make sure they machine to fit the bore of the wheels on the hubs and face the axle shafts to sit below the drum face on the rear.
Or you can try your luck with these....