I may, or may not, depending on funds, be able to score a holley terminator EFI for my 440 build. If I do, I am wondering if there is any issues with using it for the initial break in?

Its a basic build. 10.5:1 compression, aluminum heads, stock stroke. cam is 254 duration @50

It should be fine. Your biggest concern will be timing.

You will be fine. I have a Holley Terminator on my 71 RR and it was fine on intial start up. It controls back to target AFR even before it "learns". You can also set the idle to the desired RPM's for the cam break in which makes it even easier.

I have a locked out distributor from another build that I had to sell, I think that will make controlling timing easier

If the base timing is set correctly and no wiring problems it should fire right up.

Flat tappet cam and is the system USED and already run on another engine ??

Me , if it's a flat tappet cam I would do it with a carb and then swap on the EFI ...

Yes flat tappet cam. 56 lift with 1.6 rockers

Not used, brand new

It can be done... I broke one in with the non-learning type. Suggestions are 1) Double and triple check your base inputs to make sure they are right 2) Set timing as close as possible... like a carb break in