I have a set of Maximus Merc Labbed 28p props I plan on using with a Donzi 38zx. They look to have the long tube. From what I have read the short tube would be better for bow lift which I need. How much of the tube should be cut off and what is the best way to cut it?

The tube was never intended to be as long as it is. When we first developed that propeller back in 2001 the diameter was 15.25" and the tube was just like the Bravo 1. The development was a combined effort between us at Mercury Racing and the main plant Merc Propeller Co.(they used to be called that). So the prop became 15 5/8" diameter high raked and a long exhaust tube. We began cutting the tubes down immediately when Labbing the props due to a variety of performance issues. The tubes were made long to aid in the manufacturing process, not for performance reasons.
A number of years ago Merc Racing decided to market the shorter tube as a another way to create a new product and sell another option. It is just a tweak we do when useful.

My suggestion would be to make sure the boat gets on plane easily with the long tube before you start cutting the tube down. We tend to cut the tube in 1/4" increments. 1" is common for us in a Labbing process.
Also, the recipe for Labbing changed on these props in the 2006 ish time frame at Merc Racing. They did it for a few years to expedite the process. The performance suffered and they finally returned to the process Dennis and I developed in 2003.
I have written tons about this subject in past threads in this forum over the years if you scroll back.
Call me if you wish to discuss this more.

Brett