I hate to be negative (in a manner of speaking), but for a once-in-a-lifetime shot I think I'd reconsider using T-55 and use something more like a "real" sheet film. If the OP had more T-55 experience I wouldn't be so strong in this recommendation, but I remember that it took a while to get proficient with T-55. Exposure took time, development took time, and clearing took time before I felt I could take one shot and be successful. Maybe I was a slow learner???

Ann makes a good point that underdevelopment will make any exposure not very good.