M725 Carburetor

Truck runs fine at idle as soon as I let the clutch out and start moving I start to get some back firing. Once on the road shifting in to higher gears causes the same issue. I saw a thread on the needle valve should be #70.
The carburetor has been rebuilt, distributer cap is less than ideal, original with coil removed to the outside and a hole for the wire in the cap. Where might I find a new old stock cap? thanks in advance.

Are you looking for an inner cap or outer cap ?

Once you get good distributor parts. Check your timing. The stock engine will run with crazy timing settings at low speed and then protest in weird ways at higher rpm.