i have a 99 gs and was wondering if there is ANY possiblity to have a tank mounted petcock more like the katanas i understand it could interfer with the frame but i am just looking for altenatives i have found some factory tank valves that are vacuum operated but the hole spacing to mount it is to big for my tank the spacing is 34 mm..... any kind of advise would be welcomed

Lets first address the why of the matter...there may be better alternatives than what you're considering.

So...why do you want to do this?

Have a look here for universal pecocks and the correct fitting:
http://www.partsnmore.com/parts/suzuki/gs500e/#product-08-0446

Well to answer why I want to do this I'm the type of person the believes in the acronym K.I.S.S. Keep it simple stupid lol the way the fuel system is designed on these bikes is overly complicated it shouldn't be this much trouble to feed 2 carbs. And thanks for the link I will check that when I get off this evening

Years ago people on here were swapping in Honda CRF petcocks.  Never tried it cause I don't see the point.  "If it ain't broke, don't fix it"   

-Jessie