For a stock 351 3"id is too big and of little muffling value. Check out reverse flow chambered mufflers or cobra pack 2 1/2" spiral louvered mufflers.
You can get the spiral louver made here in Australia to whatever length you want.
Too big is not always good you will want to keep the gas velocity up to assist in extracting the next pulse.
Check out this for some ideas.
http://www.classicchambered.com/clas...cts.html#cobra