There is a way you can work out the load rating of the axles, i found this when i was researching the axles for the gemini. Keep in mind these are billet axles so they are probably higher load ratings than a stock axle ... especially after years of use in a car!
for the record, stock gemini axles are 23 spline
i was amazed at the jump in load rating from 28 to 31 spline, especially when the axle itself is only a small increase in diameter.
from this website:
http://www.altra9.com.au/altra_9_flanged_axles.html
A9 FLANGED AXLE TORQUE RATINGS
23 Spline – 2875 ft-lb (per axle)
25 Spline – 3750 ft-lb (per axle)
28 Spline – 4800 ft-lb (per axle)
31 Spline – 7000 ft-lb (per axle)
35 Spline – 9600 ft-lb (per axle)
FORMULA FOR AXLE TORQUE
(Max torque of your car (ft-lb) x (gearbox first gear ratio) x (diff ratio) x 90% efficiency.
Each axle should be able to handle all of the maximum torque your car can produce. This will create a 200% safety factor.
Example: A car has 600 ft-lb of torque, Turbo 350 transmission (2.52 ratio 1st gear) and a 9" diff (4.11 ratio gears)
Calcuated as: (600) x (2.52) x (4.11) x 90% = 5592 ft-lb
Therefore it is recommended that you use a 31 Spline axle which has a Torque capacity of 7000 ft-lb
so in my case, the last dyno i had done on the 202 was 260 ft lbs of torque was the peak, first gear in the gemini of 1.82 and 3.45 diff ... now the one thing i was guessing was increase in performance from more compression and methanol .... i have heard people say you get 30-40% increase in HP from it ... so i went on the safe side and added 50% more torque.
390 ft lb x 1.82 x 3.45 x 90% = 2204 ft lbs
so as you can see 28 spline axles should be able to handle the power my car will potentially make .... keeping in mind the input shaft spline on a borg warner diff is 28 spline too, so in theory it should fail before 28 spline axles do! but i went 31 spline cause you can get full spools for BW in 31 spline and theres bugger all price difference!