I won't waste my time, effort or money on a girdle as they don't add any strength to the block main webbing, which is real weak on the 440 blocks
By using a good 400 block and wanting to make the bottom end stronger with after market main caps I use aluminum now as I can't buy ductile iron caps anymore
I haven't and won't use steel main caps as I'm afraid it will break the main webbing by being stronger than the block is, so the weakest part fails first when stressed to the breaking point
Most aluminum main caps used with good main studs will absorb a lot of movement and vibrations from detonation before being permanently damaged
Good luck on your build