There are varying grades of hot:-
1) Faint sense of warmth when cover is lifted.
2) Hand feels warmth when gently placed on top.
3) Part is very warm but hand can be left on top.
4) To warm to leave hand on top.
5) Too hot to place hand on.
6) Too hot to touch.
7) Smell of overheated material.
8) Smell of burning material.
9) Some smoke noticed.
10) Lots of smoke.
11) Sparks or flames.
Could you give us a sense of exactly how hot it is getting?
Have you tried (with the engine off) holding your hand over varying places and parts
round the stuffing box to get a sense of how hot they are?
Since the problem has only occurred since the new engine was installed possible causes that come to mind are:-
1) Slight misalignment of propeller
2) Slight misalignment of shaft from V-drive
shaft has moved during engine installation
and an existing groove/ridge is now being pushed against the stuffing box housing. If this is the case the V-drive may be just a little warmer.
Don't forget you have close to doubled (maybe trebled or more given the condition of the Leyland) your power so any fault that would not have been noticed with the old engine is now very visible.
If the drive train can be realigned so the propeller shaft is moved slightly aft (0.5mm could do the trick!) it may help if the problem is the propeller shaft rubbing against the stuffing box.
There is a significant risk in continuing with a "hot" stuffing box, but immediate repairs
are likely to be very expensive.
Bet the boat goes great with all that power.