Gilby,
You can buy retro California plates here:
http://www.licenseplates.tv/ Not sure if they're really legal but I have seen the old black and yellow style vanity plates on Cobras.
I have an ERA with sidepipes so I'm not sure if your slip fitting connection is the same as mine. I tried about 1/2 dozen different sealers trying to get the slip fit joint to stop leaking. None of the sealers would last more than a month or two before burning up or blowing out. I finally permanently fixed the slip fit connection. I did it by welding a 1 inch long piece of stainless steel tubing (about 1/2 OD and 5/16 ID) perpendicular to the length of the sidepipe right at the connection to the header. I then cut a notch through the stainless tubing and the sidepipe. I then inserted a 1/4 bolt through the stainless steel tubing. After slipping the header and the sidepipe together, you can tighten down the nut and bolt effectively clamping and sealing the sidepipe to the header. I've got about 3000 miles on the car since making the change and it works great. No more popping and backfiring when I let off the gas.
Chris