Hello, I also have a 1958 MGA, however mine is a coupe. There is a chassis number that is stamped on the right hand chassis cross member just in front of the passenger seat. It is a 5 digit number with a stamping approximately 3/16 inch high. The problem is that the carpet sits right on top of that cross member and water gets trapped in the carpet and corrodes away the number. You just might be lucky and be able to read it. The numbers appear to be hand stamped and as on my car, when they stamped the chassis number, it created an indentation in the top of the cross member. Try wiping your hand across the top of the cross member, and if you feel an indentation, look very, very closely for the number - it is not a deep stamping. If you find the chassis number, that's great...then you can easily get a Heritage certificate that can get you the entire car number that would be stamped on the car ID plate.
If you can't find the chassis number, there is also a body number stamped on a plate which is spot welded to the right hand side of the firewall. That number may be of some help in trying to get a Heritage certificate. I sure do wish that MG had the foresight and stamped the chassis number in a place not likely to rot away.