da4head - although the castor blocks and front steering hubs come as a set, the hubs themselves do not have any castor built into them. The castor is defined by the castor block. I guess I'm curious as to why you'd want to use AE C-hubs on your Losi. Heck, even some of the AE guys use Losi hubs on their car.
Internal Drive Ratio - the formula for internal drive ratio is pretty simple. Take the number of teeth on the diff pully (41 or 42 on the Losi) and divide it by the topshaft/layshaft gear (23 on the Losi) and this will give you the internal ratio of the car.
42/23 = 1.8260.....
41/23 = 1.7826...
In the case of overdrive using a 41 front and 42 in back, about as close as you are going to get is to use 41.5/23 = 1.8043... But this isn't necessarily accurate because of how each axle rotates at a different speed. If the front is definitely pulling and the rear has to scrub to keep up, then the 41/23 is probably closer to accurate. I think the difference in internal ratios is small enough to just use 41/23 when using overdrive.