[mr48chev]

If it doesn't have an alternator that is feeding back which is a way too common thing on swaps you have a wire that is either hooked up in the wrong place or you have a hot wire crossing the ignition wire.

I'd do this first: start it and then when you shut the key off and remove it and it still runs get under the dash and pull the ignition wire off the switch. That will tell you if there is power getting to the ignition from somewhere else. If it quits it is at the switch if it doesn't quit it is somewhere else.

Or just get your multi meter out and see if you have power to the + side of the coil as it sits there with the key off, out and in your pocket. If you have power there go in and unplug the ignition wire from the switch and check to see if you have power to that wire from another source. If you do, find out why.