Patriot: 8 grns of Unique under a 250 gn SWC (keith style bullet) is a book accuracy load at about 830'/sec. I would use the same charge with a 255 gn swc. Most of your 45 caliber cartridges, the acp, auto rim and colt have the same barrel twist, 1:16, and from my older Lyman manual seem to get good accuracy from cast bullets at around 830-850'/sec.
You may not get that kind of accuracy from a fmj 45 auto slug as an accuarcy load for that bullet is 3.5 gns of Bullseye for only 622'/sec. I fully believe you can attain better accuracy from a cast bullet than from a jacketed one, or at least as good if not better accuracy.
For your seating problem with those 250 gn speers - you may need to bell the case mouth a bit more to get the bullet to seat more easily but yes, they are made of swaged lead and are softer nosed than a cast bullet is so you need to be careful with that. HTH. Mikey.