A 200 gr bullet and 4.0 gr of Bullseye will function most if not all .45 ACPs. That was my gunsmith's test load until he changed to N310.
So go ahead and start there, just don't load up a big supply before you have tried say two magazines worth.
1.265" is long for a SWC. The plunk test will guide you. All that means is to drop a round into the chamber of the barrel, clean and out of the gun and listen for the sound of the case mouth hitting the chamber mouth. Also see the case head flush or slightly below the barrel hood. Compare with a factory load.
I think you will end up at 1.24-1.25" with the long nosed SWC.
The length from the casehead to the SWC shoulder just in front of the case mouth will likely be 0.92-0.93".
You need enough taper crimp to eliminate the seating flare and not so much as to cut through the bullet plating. Not as critical as usually described.