Does 8mm spark plug wires make a difference than 7mm on a dodge ram 1500 v6

The short answer is no. You can go larger than stock with no adverse effects but you cannot go down. The biggest thing to look out for is the quality as some aftermarket wires (even from reputable companies) can cause issues in the near future.

Larger diameter wires have better insulation which might help if you install a high energy coil. I see no benefit for a stock vehicle.

I think the only upside with larger diameter wires is that they may last longer before the insulation starts breaking down. Also a 7mm quality wire would be better than an 8mm poor quality wire.