Here is what I used on my last TF P-51 build. http://www.robart.com/retracts/6-12lb510.aspx
6 - 12 lb. 510 Series SpringDown Pneumatic Retracts these are strut ready and you will have to buy them seperately. Its a very neat setup with a small air tank thats only used to pull the gear up, not down. Plus there CNC machined Alum instead of molded plastic that can and does break. I'm not sure if Spring Air is still around or not I thought I was told they went under several years back, but maybe not. In my book Robart has made a big hit with the new line of retracts they now offer, I see they now have the 90 Degree rotating ones too for the Corsairs and P-40. Those old 615's had there issues to say the least.