Timing light, vacuum gauge, and volt/ohm/rpm/ meter are a must have if your tuning/trouble shooting yourself.
There's no way around it unless you've tuned the same combo's for years.
Even then, you'll find them the most valuble tools you have.
And you wont be using the "wag' method.

Back firing under wot(wide open throttle) is "usually" timing and or secondaries opening to fast.
Leans out fuel mixure, then "fuel dump" fires up inside intake.
The secondary air flap can be adjusted to slow down/speed up air flow. Theres a tension spring and screw on one side.
Tightening it will make "air flap" open slower/later.
Usually what needs to be done.

The 4 lean/rich plugs...?
Could be the open vacuum port.
But more likely bad fuel distribution from carb.
Do you have another to swap/test?
I aways had a known good spare(same one) since 79 just for that purpose.

Hook up distributor to full manifold vacuum.
But be sure to disconnect and plug it at intake/carb port when checking initial timing.
As for setting...thats a tuff one.
Depending on cam and degree install, could be anywhere from 8-18 initial, and 32-40 total.
OH, and SET TIMING BEFORE final adjusting carb.

You need vacuum gauge and rpm meter when ajusting carb.
Try for highest "steady" vacuum reading at lowest idle(within reason). EVERY engine is alittle diff.