If the cam is truly 280 at .050 intake duration, you will need a very high stall converter. A converter said to be 5000 stall may be very close, but it depends on the engine torque. A big block will stall the same converter much higher than a small block. In your case, if you upgrade all the parts around the cam, and are willing to run it at the right rpm, then you will need AT LEAST a 5,000 stall, but only normally aspirated. The more nitrous you put in the motor, the tighter the converter needs to be to get back to the right stall rpm. AS long as you are only putting in a 75 hp shot, it is only adding about 100 ft lbs of torque to the combo if you start it at say 4,000 rpm. The higher you add the nitrous, the lower the torque the motor sees. The lower you start it, the more torque it sees.
A general rule of thumb on picking a converter is pick one that will stall about 1300 rpm below the shift point. So if you are willing to set the motor up to run 6500 rpm, then 5200 rpm flash stall behind your motor will usually give you the best results. The cam you have probably has an rpm range of 5,000 to about 6800 rpm with a single plane intake, the right headers, etc.
There are a lot more things to consider though in the tranny. Is this a reverse manual race trans? If not it should be, with the right clutch drum, better sprag, stronger band strut, among some of the things that make them work, and work safely.
One last thought. If those rods are good, then you shouldn't have much trouble at 6500 rpm or less, normally aspirated. You state you are confident in a 5500 rpm shift point. Your cam is designed for a lot more, and the torque comes in from about 4600 rpm on up. That is why you have been OK using the nitrous down low so far, it covered up the lack of torque down there. A 75 hp shot may be adding as much as 200 ft lbs of torque at the stall of the stock converter. If you activate nitrous at very low rpm, keep in mind that is what scatters engines, because the cylinder pressure goes WAY up with the right cam. You are getting away with it because you have the wrong cam, with way too much duration for that rpm, and not much cylinder pressure down there as a result.