Check it does what it should, i.e. you can get lots of advance, it's jetted properly, carbs balanced etc. You may need different ignition (electronic) to get the right advance, I can't recall what that is, sorry.
NB: unless it's been modified it'll not put out 135 brake at all that number is shall we say optimistic
Add 3d ignition
SPC long ratio first gearbox or similar
The box is probably the biggest benefit, mappable ignition won't give you more power
Other than that it's engine change time and or lose some weight somewhere / remove the screen for more speed. You might even find a tuned XF somewhere for less money than an engine swap if that's an issue?