13's, stock '71 318 short block.

I would say you should be pretty close to 13s or atleast low 14s with that combo. I had a 74 swinger w/stock block,heads with bolt ons similar to rumblefish would run 14.7s on street tires, 2.71 open rear,stock converter and weighed 3150# it was a dog off the line but it pulled pretty good til about 5000rpm. With 4.10 gears and a gutted interior you should be close as long as the shortblock is healthy.