Just want to double check something. You said a 40 size P-40? I think the CMP is a 50 size P-40. If this is so, then AK Models has the retracts designed for that bird.
http://ak-models.com/mm5/merchant.mv..._Code=Retracts

Fair warning, those retracts are in a plastic or resin case. I broke a set on a hard landing. I have the CMP Corsair 50 and I finally gave up on the cheapo retracts and installed a set of pnuematic retracts. But for this budget bird, it is a little odd spending $250 for a set of Robart 615s when you factor in the 615s, the air kit, and accessories. That is before you spend $80 on a set of robo struts if you choose to use them. I would not have installed them had I not already had an old set of Rom retracts.

I also had a problem getting the aircraft to settle down without nosing over. I finally gave up and installed the fixed gear and bent them way forward. That finally worked, but it was a shame to have fixed gear on such a nicely finished airframe. The pictures show the original broken retract. Then the second is the installed air retracts.

Bob