Should I get the largest-diameter primary pipe header available?
The Viper cylinders are not huge. A 488cid V-10 is comparable to a 390cid V-8 or 195cid 4 cylinder. So HUGE primaries would not be called for, unless other engine modifications were done, such as larger valves, stroker crankshaft, etc. Bigger is not always better. Throttle response should be much improved with smaller headers on a stock motor. Headers that match the D-shaped eshaust ports are a must.