Running well up to 50 under load and then falling on it's face, sounds like a fuel delivery problem. The carb float chambers are full of gas. If you take it out faster than it's put in it runs out and engine starves. This can be several things but if the float is set too low and the float chamber is almost empty it could easily run out. Didn't I post a link that shows how to set or check the fuel level???? I think you use a clear piece of vinyl tubing.
No doesn't always backfire.
Yes uneven float level changes everything.
Take for a drive (if possible) get it warmed up well and pull over, don't let it idle.
Pull plugs. They should be evenly colored at least. Darker or even black is too rich. Light or tan is about right. Very light or even spotlessly clean white porcelain is way too lean. You may notice the front 3 much different from the rear 3.