Can any one tell me what the measurement from the back of the front axle to the motor plate should be using the 2.5" axle with the GF1 SRM 87.5/40 chassis

In most cases, it would be what's left, if you are 40" center of rear, to motorplate, then what's left should be to the center of the front axle, or 47.5", but this is negotiable, it's more or less where it fits in best, then you can change it, if you want a different feel out of the car, both front and rear can be changed, not much, but there is usually a fudge factor built in. Bob

Yep-minus 1 1/4" (half the front axle diameter).