Are your new mufflers less restrictive than the old ones?
If so, that might be why it did not backfire with the more restrictive old mufflers.
Tweaking the idle mixture screws sounds like a good idea to me. If that doesn't work, tweak the timing.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission