I have a 85 K5 350/700r4/np208 How would I go about putting duel exhaust under her.

 

You can go online and order dual exhaust kits from Summit Racing for $132. I have the link below. If you don't want to order new exhaust you can always buy exhaust pip and D-I-Y.
https://www.summitracing.com/parts/w...3CUaAmtV8P8HAQ

 

On my 88 K5 I bought a Magnaflow system. It's single 3" pipe and the system on it is a Magnaflow, but no one makes dedicated exhaust systems for these 28 year old trucks. So I bought the new magnaflow just to have for when I do go about swapping a new exhaust on my truck.