Backfiring?

[JOZW30]

Recently completed the rear posi gear swap (2.73:1 to 3.73:1) in my 72 W30 4 speed & everything seems great mechanically.

I have about 150 miles on it and am trying to be gentle until it's had some time to break in. But on the couple of "hard pulls" I've tried, I have noticed that with the revs high, and the clutch engaged (first to second shift), I am getting what I think is a slight backfire or popping.

This never happened before the gear change. Do I have to adjust my timing now? What exactly is the cause?

Am I just over-revving now and need to get used to the new set up?

And, is this harmful to the engine?

Thanks!

Joe