Pay attention to the keepers. The exhaust valves on the 40s have rotators on them. Some kits will give you the wrong sping kit and you will have the ever popular problem of the rocker trunion hitting the exhaust valve stem....
I am deciding between my stock sd ho cam or a mild aftermarket one.
I know what you are saying about the rotators, the factory gt-40's have diffrent length springs on the intake and exaust. I am looking to save cash and reuse the locs and retainers.
So I guess I am looking for springs with an id no smaller that .97", OD in the 1.45 range, that are setup for the rotaters with shorter length on the exaust and longer on the intake, can handle up to .54 lift (some saftly margin I probibly wouln't go over .51) cam, and have enough seat pressure for a roller cam.
I know I am not the only person that has looked for this.
We used some Crane 99837-16 springs with a Crane 2031 cam(now pt 444225) 1.7" Crane Cobra rockers and had no issues at all. I believe we used a 99943 and a 99936 retainer, 99041 lock.
Once you know what cam you are going to use, then we can pick the components out.... Locks and retainers are cheap, I'd replace them along with the vavle seals.
When doing some research on this subject, here's what I found from my machinist. We looked at Comp Cams 842 series. They would have to be machined to fit properly on the GT40 heads. We ended up using the TFS 200 kit, because I was using 1.7 RR's and a TFS Stage 1 cam. No machining required and fit perfect. I understand the TFS 100 kit works well too, just depends on what your requirements are.