are the mounts for the old tanks there still, if so try to use those. other wise yah attach a couple of 2x4s to the stringers glass them down and you can attach the tank mounts to them a # of diff ways.

if you check the thread just below this one i have the same boat and it still has the original fiberglass tanks. the way they made them was they made one thick piece of glass that runs across the sides of the hull and bonded a fiberglass tub to that then glassed the tanks in at the bottom three sides. if i take my tanks out i am leaving that thick piece and the 2 ends and having aluminum tanks made to fit between that so it will keep the tanks located and have something to mount them too. would like to see some pics of your challenger.

very cool what year is yours and what are your plans as far as power and jet.

http://www.socaljetboats.com/hulls/saddle-tank-mounting/msg156748/#msg156748

Here's a thread from April 2011. I posted an article Hot Boat had about mounting saddle tanks.

not to throw a wrench into the mix but maybe you whould want a bow tank? my 74 kona came stock with one.i always liked the idea better .