Spark plugs going bad could cause it to miss at high speed but

NOT backfire. Check the inside of your distributor cap and make

sure it doesn't have any tracks between the individual contacts

(which could cause a spark plug to fire on overlap and backfire

either through the carb or exhaust)

Another possibility, especially if you are using original

lifters, is that you have wiped out a lobe on your cam.

This happened to me on a 67 122 and also on two different

B30 engines. Quick way to check is to pull off the

valve cover and using a ruler from the top head surface

to the retainer, have someone turn the engine over slowly

(use a wrench on the front crankshaft bolt) check the lift

of each valve. All should open 3/8" or more. If you find

one that doesn't open, there's your answer. I've been using

IPD lifter and pushrod kits and have eliminated such problems.

(of course if the cam is bad you gotta replace it.)

Under no circumstances reinstall a worn OEM lifter - one

that is not perfectly flat and shiny on the wearing surface.

You will only eat up the next cam.