new 496, everything new. went through cam breakin, driven about 75 miles. car ran prefectly. next day when I started it had a backfire in carb. started engine and ever since have a constant backfire in exhaust. local shop said power valve in holley, replaced it, checked wires no crossed wires, pulled plugs all look good, ran compression all at 160. thankful for any ideas to check next.

when does it backfire,at idle,steady cruise,under exceleration?
2 things come to mind.either theres an exhaust valve hanging open or spark isnt firing or at the right time in one or more cylinders.
Bad plug wire or plug.I would put a set of new plugs in it,kind of sounds like one got fouled somehow.If not check the plug wires to see if they got burned on a header and for ohms?

all plugs and wires are new

When does it backfire???
if it gets worse when you are asking for some power from the engine it would most likly be in the ignition system.
a new plug can fail in short order.
simple compared to:
putting each cylinder on DTC of the compression stroke and pumping a little air in and listening at the exhaust pipe for a leak.
if you dont hear anything after doing all 8 then try the ignition system.
I may be wrong but i dont see how a carb problem would cause a constant backfire out the exhaust?

just reread.
you did a compression test so there cant be a valve problem.
plugs,theres at least one that has failed.

Just because spark plugs are new, does not mean that they are good. It can now be fouled,the box may have been dropped at the supply store before you bought them, which in turns may means damage,or may now have a closed gap. A bad carb/float adj. fouling out plugs may also be a consideration.

Check inside your distributor cap. Spray some wd-40 in there and check for carbon tracking. Make sure your mechanical sdvance is not sticking. all parts should move freely.
Run the engine in a very dark place and see if you can see any spark jumping around.
An exhaust valve set a little too tight could be it. Was it hot when you did the compression test. It could be set close enough to seat when the engine is cold. Possibly a sticking exhaust valve. Pull the wires one at a time and see if it changes when a particular wire is pulled.
Double check your firing order and see that no two wires are running parallel and touching each other. Enough current can be induced from one wire to the other to fire the wrong cylinder.
Squido