Backfires on shifting, muffler has a huge hole in it, Are the two related? Options

[gollygwagen]

I'm sorry to keep posting with silly newb posts, but I wasn't finding a clear answer. The car backfires sometimes when I shift gears and when I coast with it in gear. I think the second is likely related to the massive gaping hole in my muffler I plan to replace today, but if it wasn't I suspect that I will quickly blow a hole in the new muffler so I wanted to ask.

Car is coming together with all the stuff I replaced and next weekend I should replace all the fueling parts from the tank back and finish replacing the ignition pieces. After that it's mechanical and since it runs pretty well I'm thinking that's OK for now, just need to find the fancy oil this week. I know a local racing store carries Redline and Royal Purple products so I'll check with them today. How many quarts will I need and is the oil change interval similar to normal cars or due to it's air/oil cooled nature do you want to change it more often?

You guys have been great. I drove it to work for the first time today and saw three 911's on the short 7 mile stretch of highway I drive. Normally it's only Cayenne's, so I think it's a sign I'm on the right track.