Chevy & GMC Vans

I want to haul Kayaks with my asto van on top, but I dont have the racks, I live in Houston so I think I can find them at a junk yard fairly easy. Is it hard to do, how are they attached? with sheet metal screws or are they bolted on?
I figure i can use the holds to drill the holes i need but can anyone tell me how much of a pain in the ass it is going to be and will it leak and will it hold up to hauling kayaks?

Ive used Yakima racks for both my Astro's. My '94 had a stock luggage rack with yakima lowriders attached to that. It worked well for a tandem kayak and full length cargo box, side by side. My '95 didnt have a luggage rack. So I installed yakima trackrails directly to the roof,which is lower profile and the better of the 2 options. In my opinion your better off going with the tracks over the luggage rack.

I've had a few leaky stock roof racks...
Find a better way IMHO.