Redpep
The Speer 300 grain Gold Dot is no longer made in .451 - they changed the nose configuration it 2 years ago and it now made as a .452. They (speer) does offer a .451/300 grain but it is not a Gold Dot, it is a Uni Core bullet. The .458/300 grain is also a Uni-Core bullet.
45 Revolver UCSP 45 Colt HV
45 cal. Flat Nose HP (45-70; 450 Marlin)
This is the Gold Dot...
45 Revolver Golt Dot HP 454 Casull
I can not find the listed velocities on the Speer site but you can look up the velocities of the gun suggested and come up with a good idea.
Here is Gold Dot spec sheet
Hope all of this posts....