A a stock with a parallel comb offers some benefits but it is hardly necessary. A stock with the comb at the right height is VERY important and any way to get it to that height will work just fine.
Trap stocks usually have alightly higher combs on them so they will shoot slightly higher than sporting guns.
My advice is to have your 686 fitted to you, consider a 4-way adjustable comb and shoot it. In my opinion, investing money to have a barrel fitted to another receiver is money poorly invested. Just modify your 686 slightly and you will be fine.