Exhaust help

[Security guy]

I've got a couple questions for you guys. I've got a '71 short bed with a stock 402 and mid-length headers. Will 2 1/2" exhaust pipe be fine or do I need 3"? Whats the difference? Loudness or horsepower/torque? This will be used for cruising and a little towing but I want it to sound good also. One shop told me not to start it with the cold weather because having open headers, when you shut it off, the cold air coming back up the headers can warp the valves. Anyone heard of this? Also one shop quoted me around $400 for a full exhaust out the back with flowmasters and another shop quoted me about $250 with incenerator (flowmaster wannabe's) mufflers. Is it worth the $400 for flowmasters and does the price sound about right? Thanks for the help.