I have used a Big Bad Bow, Ace #49, and nite finder spring in my longshots with great results. I've heard of the Mcmaster-Carr k18 spring working well too. Just make sure to reinforce the boltsled no matter the spring.
What is the best spring I can put in without having to reinforce it? I do not want to get some insane range, just about stock range basically. The current spring is worn out and I just want to restore it to working order.