Basically, if it is a one piece gear, the gear is flush mounted against the bottom of the frame (usually with a pad for wear and cushioning) and clamped in place with some retaining brackets. Of course, you have to have some fixtures welded in to accept the clamp bolts, and you want it in a cluster area where the fuselage has the needed strength. You might also try searching for Cassutt construction photos... same type of attachment.
If your gear legs are two separate pieces like some of the Cessna's have, then you need to build a receptacle box for each leg. The best example of that might be a disassembles Cessna, then just work out a way to bolt the receptacle box to the tube frame.
Find someone with a newer Citabria, the ones with spring-steel legs, and see if you can get a look when the airplane is in for inspection and the forward belly panel is off. There are two legs and the truss to support them is pretty stout. They also use NAS bolts, not AN bolts, since the loads are concentrated and fairly high.
American Champion stopped using steel legs a few years ago and you can't even buy them now. They are replaced with aluminum legs, thicker but still lighter, and not so crack-prone as the steel. We had a steel leg fail and bust a wing. Cessna has also recently come out with extra inspection demands on their legs, as they're getting reports of serious corrosion pitting and probably some failures. If you're going to use Cessna legs, find good ones.
The Christavia MK1V has a 2 pc cessna gear (150).attached to steel tube fuseledge.I'm presently building that type of aircraft and have installed a Grove 1 piece aluminum spring gear. Does this setup interest you?