ok, with the valve spring and retainer installed is about .960 you say. so you will need a spring that is close to that height. I believe there some springs out there for spring heights at .950 also got to remember you height may change little with the new retainers and valve job. so need to consider that too.
ole4 tried posting a spring height chart that might help with your project. I don't think that wet through?
either way a 26 pound single spring you will need for a 265-280 lift you say you have.
I still think the factory yellow spring might be something to look into. its only a single spring so I wouldn't think it's rated higher then 26 pounds?
even if it's rated little bit lower then 26 and it doesn't any other problems .for example coil bind. it should maybe work. hey it was made for that engine and it's not likely your going to float the valve at 5000rpm's. most web sites that sell springs don't always show all there springs and spring heights. so your probably will have to call them and ask.