You can boot up the performance of the tails filling completely by using a small jewelers file to open up the vent at the end of the tail on the vented side AND to add vents in the same location on the tails on the un-vented side. These thin appendages have to be able to breathe, especially in one the size of an 8" Mo's bait. You'll end up with tiny tags of plastic at the vent site on the finished tails but they pop right off and are a whole lot more enjoyable than a half a tail. Given the flow rate of hard plastic, even a bait this large I would think shooting a hard plastic in it would be nothing but headaches, especially in the tail. The first thing I'd do is go to a soft plastic to see if the baits turn out and work from there.
Nominal clamp pressure is needed at the tail portion of the mold on both the 2 1/2" and the 3" Mo's baits using soft plastic that's shot quite hot. With the size of the mold, the mass of the mold, I'd probably worry about warping the mold by heating it with a torch. A couple minutes in a warm oven to get the chill off is preferable but then some warm-up shots with junk plastic is always the best way to pre-heat a mold.