Is bigger tyres worth it?

Was testing my 69 dart this today and have no traction. I have about 350-400 hp 360 with 727 and 8-3/4 rear end with 3:23 diff. I have 15x7 steelies rear and 235/55 street tyres.. these are old so anyway I need some slicks or MT drag radials. But my question is how big of a tyre do i need to get good traction? Do i need to put out and upgrade my whole rear suspension from stock to fit 275s or do i get away with 235s wich is the most i will fit without rubbing. Sorry for bad inglesh i am from norway.. lol