If HEI, pull dist cap off pull rotor, turn it over and see if there is a a black burn mark on bottom if there is the rotor is bad.
Also if carburated the carb could be going bad and backfire can be caused by lean burn , ( not enough fuel)
could be sticky valve, or wrong valve clearance, broken valve spring
You might also lubricate the centrifugal advance counter weight pins, use a small amount of white lubricate on the pins, make sure that the pins are not worn, check for sticking.
backfiring is a sympton of a lean condition. look for vacuum leaks and fuel delivery problems.
The valve timing is offAnswersounds like a possible worn camshaft lobe. a fairly common problem on late 70's Chevy small blocks
Rpm needs to be set at 800 - 900 .. that is factory setting.
I belive it is 4 degrees advanced at 500 rpm
an idle rpm for a 350 stadard trans. is about 500 rpms. an auto is about 750 rpms. this is if the eng. has a stock cam. if not it may vary
Standard transmision, 750 rpm. Automatic transmission, 600 rpm in drive.
1500 at first until it kicks down to 850 rpm
The Chevy 350 is a V8 5.7 L engines which yields the horsepower of 200 @ 4000 RPM and torque is 310 ft/lbs @ 2400 RPM. It has designed by General Motors and started manufactured by 1968 until now with enhancing its power time to time. To know more about Chevy 350 and its complete description you can go through www.hiperformer.com/chevy350/overview.html. I refer this page for gaining information for my Chevy 350
The practical limit is 6500 rpm. Above that and you risk parts failure.