300 is possible, but pretty much stupid. I know guys that have been using a 200 Shot for a long time on there high milage engines similar to yours without a problem. Of course it depends alot on the driver technique as well. A driver who dosn't know how to use nitrous will easily blow a hole in a piston or something else on his first try, even with a small shot of Nitrous. Overall I would play it smart and stick with a 150 shot, and insure you know how to proparbly use it. You failed to mention what you have for a fuel pump, I imagine you have upgraded that as well. If not, it would definetly be smart to do so, especially when nitrous is in the plans.