here may be your seat solution ...
Boxster/996 seat mount - PRECISION CHASSIS**** WORKS************
i'm working on the manual steering ...
http://986forum.com/forums/general-discussions/31007-steering-rack.html
specifically, this is the 'proper way':
Flyin' Miata : Tech: 1.8 conversion
but also shows the sneaky method where you just revise the plumbing by looping the hoses from each side of the piston. belt routing is fine unless you have a/c delete as well, in which case you need the $250 idler pulley from bgb.
can't help you with the rest, but I wish the aftermarket offered:
- a lightweight cayman roof shell with lexan windows and without operable hatch that attached at the front latch, spinlocks and rear trunk latch. 1 min to remove, complete engine access, under 25 lbs.
- a rear speedster panel/manual top like on the spyder.