Easiest thing to do is to install a 1989 through 1992 Allante' 4.5. You have to use all the original emissions equip and modules. Footprint is exactly the same. Exhaust manifolds, EGR valve, diverter valve, O2 sensors, and upper intake off old engine all need to be reused. I've never tried to put another non-Allante' Cadillac engine into an Allante'. I know the lower intake is different on Allante's that other Cad's and that is work to swap. Not sure about the heads.
If you're looking for more power (maybe +45 hp), go with a 4.9L cadillac engine and swap over the Allante specific parts including heads and intake manifold. The extra power however might speed up the demise of the F7 tranny which was not known to be that strong to begin with.
If you can invest more money and want something with a lot of power and unique, install a transverse 5.3L chevy engine. It was used in the newer SS Impalas and some other models as well. With this engine, your car will be slightly faster than a 93 N* Allante.