Kevin, I don't have the plans in front of me but most of the planes used a Veco T-21 which I believe is a 3 oz. wedge. It's a little on the small side but the bigger problem is the venting. With the overflow sticking out of the bottom, one is sure to rip it out on landing.  In addition, when the pickup exits the front of the wedge, this also proves problematic because it's usually in the center of the leading edge. I typically re plumb the pickup exiting on the bottom and make it follow the bottom of the tank until it extends out of the front. This is soldered to the bottom of the tank. This also places the fuel line in a straight position to the needle valve without going down and then up. With so many needed modifications, it would be practical to purchase a tank kit from Brodak this way you know that your tank is soldered and built correctly. The kits are offered in several different oz sizes. Tanks are another item of our hobby which is disappearing.

If you are building the Cat as the kit is, with the engine offset and the tank behind the engine, you can use a T-21 modified for uniflo. 3 oz will give you a good ride.  TS

Hi Kevin.
I’ll check my Combat Cat plans and see if I have a suitable tank kicking around. If not I can probably make one for you.

Here's a Veco tank in a Combat Cat. Fuel tube bent 90 degrees and nested into the groove in the LE. Running pressure to the ST-C35. Haven't flown it much, but didn't have any issues. I suspect it would work on suction as well if the vent was facing forward--but haven't tried that. I dislike "glued-in" tanks so put an access hatch on this one.