What size fuse are you running, and where? Assuming you blew a main battery cable fuse, the starter is drawing too much current. A bad relay would either stick or not close, its pretty improbable (but not entirely impossible) that the contacts all fused together and shorted to ground. Usually the starter testers do not put a load on them when testing. Freespinning and cranking a 11:1+ CR big block will cause the starter to draw entirely different amperages. A starter cranking a large V-8 can draw over 250 amps sometimes. If you had no plugs in it it should draw very little current, make sure the motor turns over by hand with a breaker bar. If it does the starter is done. Gotta love headers!
