I have a 97 Donzi 27ZX with a blown 555 putting out 740hp through a bravo XZ drive. It starts to porpoise at around 72-73 mph. I’ve been told this hull will rise out of the water another 2-3” at that speed causing the prop to surface, and a drive spacer will fix this. My question is, how do I determine proper spacer thickness? Also, where is the best place to get one?

Try finding some from a member here. If you can’t IMCO sells them. I used to have four of them but gave them to the gentleman who purchased my boat toast Summer. I had a AO 26 (basically a modified tunnel). I tried 1/2 inch, 1 inch, 1 1/2 inch and 2 inch spacers. It’s amazing how much they effect the attitude and speed of the boat. I would start conservative (small spacer thickness) 1/2 inch may be all you need.