What would cause the nose to go down or the back to lift up on a power boat , remembering unlike a outboard there is not trim button !! damn
Hook would be amongst the first things id be looking for! In reality slight rocker is better in a inboard but the placement of the high point of the rocker is critical ,
Next the angle of the prop shaft !! if its steeper than it is recomended it like running a outboard with the trim hard down continuously !!and as said in the previous post your cg is to far forward . Make the back go up and the front go down !!!!
Next look at the way the boat is loaded ,tanks ,batterys and what ever else you have in the way of weighty items.
All these things to be aft of the motor !!,
After all that the weight of the Engine/gearbox ??
Something to try are bags of sand and place them in the back , take the boat for a run and keep adding sand bags right in the back as close to the transom as possible till you get to where you want it to plane once you have got the boat to plane correctly take a count of the kgs that are needed to achieve that .
The imediate feeling i have is first the shaft angle
in combitation with second the weight of the motor and gearbox is the problem
Can you post pictures please of the boat sitting in the water and then all the inside stuff , also a side shot so can see the propshaft angle and where it comes out !! Pictures are worth a thousand words !! Plus to see the boat ! i have a great passion for that style of boat and have built modles whan i was a kid . Century and Chriscraft were my favorites . or just email direct to firstname.lastname@example.org