when you get your carb rebuilt make sure you put in the little check valve (ball and spring for about $8) that will save your power valve next time you backfire. backfiring through the carb can also be an exhaust lobe going bad. the combustion can't make it out of the exhaust valve because it doesn't open far enough so when the intake valve opens it goes back up the intake instead. the plastic thing on your wires was probably for abrasion resistance.

I can only suggest what has happened to me in the past so:

Check to make sure coil isnt going bad.....I had a problem with backfireing when shifting into gear and motor would die. If i bumped the throttle up right before i shifted it would go into gear no problem and run the rest of the day!

Also check to make sure the springs on the mechanical advance inside the distributor are ok. I had one broken on my MSD system. Motor ran fine but something was just not right in there till i looked under the cover! Found it in 3 pieces.

The white hard tubing looking thing.......looks to be a line that may have been to a vacume/boost gauge??? Or maybe it was a vacume line for a vacume advance distributor? Either way trace it down to see where it goes. You may just have a vacume leak right there!!!!

you need at least 9lbs of fuel pressure,and it might be a weak valve spring.....

Good thread

Sounds like you bought this boat set up the way it is. Just wanted to add that it looks like your carb is an automotive style and not the marine version. The float bowl vents are not the "J" style. Doesnt help solve your problem but its somthing to be aware of if you ever get inspected. Alot of people use them as they are about $200 less on a Holley.