My new project

Most important is compression check first. Driving with a clogged exhaust from the muffler coming apart inside will make the sparks your wife saw . If you pushed it hard to keep going on the highway you could have burned the exhaust valves.

The sparks would be the rust and pieces of the muffler getting red hot and coming out the pipe. I had a 440 that had one bad muffler. It looked like a sparkler out the one pipe. The muffler look good on the outside . When we took it off it was like a rattle can.

For this reason I always use a welded steel muffler such as Flow Masters. These mufflers do not have duel perforated pipes inside .

The sparks tell me you have a lean condition or a bad muffler. I am hoping it didn't totally clogged before you did damage to the valves if that is what happened

Did you ever notice a car at idle puts moisture out the exhaust. This plays hell on cars that never get the pipes hot or if the car sits for a while.