I have fountain 29 fever with dyno 735 hp with xr upper and thinking about a max mod phase 4 modification. I spoke with the owner and he claims his process is almost bullet proof. Has anyone ever used any of these drives? trying to make a decision IMCO vs Max Mod? any thoughts or experiences ?

Even the best modded Bravo Drives still use the Merc XR gears. The upper gears are the weak link.

Imco scx or arneson

Bravo X drive upper with an IMCO -3 lower behind 800+ HP without any problems. Have Mr Gadgets build it for you. He built mine.
When I put the M4 Procharger on it I am going to the IMCO SCX-4

What type of boat do you have this in Tinker ?

I have a 2001 26 Daytona that probably weighs about what your 29 does 4500#
The 26 is 27 ft long