i have used a chisel, its alright if you are carful but you're rite, you can easily cause damage. when you removed the old head gaskit could you see where it was leaking, it could be pretty suttle. if you cant find where it ws leaking it mite be a cracked block which is very hard to see sometimes. you say it leaks after 3 miles, what exactly is happening, a lot of steam and your engine over heats or something else?