I haven't looked very close at a 62 muffler, but I had a similar situation where the stock muffler fit in the cowl and didn't want to spend big bucks on an aftermarket muffler.

Can you drill out the baffling or pipes inside? I had a G-20 muffler where the exhaust exit pipe on the inside was capped of at the top with just a small slit toward the top for the exhaust to exit through.

I drilled a hole in the pipe inside through the engine exhaust port side of the muffler. It made a big difference in power. It got alittle louder, but not as bad as a B&B