All depends on the car. When I had my 99 Z28 with exhaust, intake and a 3200 converter it ran 12.2s on the stock street tires @ 111+mph. I have a 01 WS6 6 speed I just picked up in December for $15,900. I don't have any times on it yet. 99 Z28s can be had for 11-13K.
With due respect to my cobra which i absolutely love, if i had a couple more bucks i'd probably be driving an ls1 right now. Those cars are straight up sick when it comes to going fast in a straight line. To make a 5.0 keep up with them get nitrous or a blower if they're close to stock, both if he's moderately/heavily modded.
GO with a nice HCI or if you think its going to be time soone, freshen up the block and throw on a stroker kit. Or nitrous or blower will work for now. Me w/ a 10.8:1 compression ratio, blower/turbo is out the door for me. 150 wet shot when I'm ready for it. Gonna try and make my 12 sec daily driver into a 11 sec daily driver.