RTV isn't a brand, it's a type of silicone. Room Temperature Vulcanizing to be exact (see the RTV?).. That's like saying PVC is a brand of pipe..

Get a soft type of gasket material, there are lots of types on the market. If you're still having sealing problems, the flanges are probably mildly warped or bent and it's probably not worth it to use a gasket unless you want to block sand them so they're flatter. Weld 'em if you have to, but it shouldn't be necessary with a decent gasket material.

 

before i put my header on, i got it for free, but i sanded the bottom like fab said. then i put a piece of copper out of house wire. then i used high temp rtv. then i put grade 8 bolts in and then torqued it down to like 40-50 ft lbs. its been over a year now and i havent had a problem. i retighted them after the first night of driving.