Welcome Doug....
....to VAF. I know one guy in Atlanta, Mike Stewart who has a 540 Lyc in his -8 and the vertical performance is phenomenal and his cruise is around 225MPH, close to but not over VNE.
Seems to me that the 250 Lyc would be an easier mod because the 6 cyl needs a new engine mount.
Does the 250 HP engine fit the same mount that you now have?
Regards,