New Crate Engine 454 up and running

Try adj the valves.

did you use good wires? The 454 is susceptible to cross fire on the wires.

Drive it at night with the cover off and look for arcing.

Has anything been done to the throttle body or was it just removed from the old engine?
T B's have a problem with an internal regulator getting weak and with less fuel pressure they can backfire.
The regulator was easy to replace without removing the T B (IIRC)and was available at regular auto parts stores the last time I replaced one.

sounds ignition related to me, wires or cracked plug maybe?

I had a 1985 454 and it continually fried spark plug cables and just one bad wire would make it backfire.

An exhaust leak at the manifold will cause back firing too.

We found that putting premium gas in the tank stop the backfiring on our 1991 pace arrow, that we had put the new crated engine in. It would backfire for miles and then run just fine, but after we started using premium gas, it ran good. We did have to have a new clutch fan put on in AZ. I hope you have better luck with your 1991 than we did. Purchase price was $11,000. Added another $32,000 in repairs and remodel and Traded it in for $20,000. What a deal.

Was the distributor out of the engine and you installed it before you put it in? Set your engine back up and see if your timing is off.. It may be off a tooth or two..

Maybe you have a plug wire crossed, or the distributor timing off by one tooth. Premium fuel should always be run with these big engines. Not only will it run smoother, you'll get better mileage, it will quit detonating/exploding the fuel, and probably run cooler in the process.

Was the crate the same spec as the original engine? Same cam?