Headers help

i just installed super comp hooker headers on my 73 nova, i hear sonunds like pockets of air as i drive, the headers barely fit, i feel as if there is an air leak, they are on there tight yet i dont know how to make sure they are leaking air, how can i find out???? what else could it be????

sounds too me more like not enough backpressure causing air to suck in through your exhaust valves....whats the specs on the motor