You will make things a lot easier if you put an A3 plug in it. You have a great engine there that puts out a lot of power for the price. I wouldn't take 5 OS's for mine. You just need to get things right. The high speed was lean in the video. The low end sounded wet. See what the proper heat range plug gives you. The A3 is a game changer in the .455.

The muffler gasket will have no effect. You can run it without them if you want.

With the OS 8 plug, try easing the throttle up and down in increments so it has time to get rid of excess, unburned fuel.