get it welded, and bolt the heads on. do a leakdown test on it. small cracks wont cause the guage to lose alot of pressure so you would have to leave it overnight or something. hopefully your valvetrain is nice and holds or you wont know what's actually leaking. cracks in blocks incaseyou dont know, can cause the car to suck air into the combustion chamber making the car run lean. if the crack is near the water jacket, it could possibly make the car leak antifreeze. i have seen a couple snowmobile engines with distinct cracks in teh block but behind the crack was uncracked aluminum, and they ran fine. of course, the crack would get worse over time and eventually destroy something.
