Yes that is a 135 degree orientation and is pretty common method of running the muffler through a tunnel works great and will not cause syphons as the carb is sufficiently high.

Yes use the tank exactly as it is in your photos. Right up aginst the firewall sounds like you have a perfect tank to carb relationship. And no you will not be adding to much weight to the nose. Remember when you check and adjust the CG do so with the fuel tank empty.

John