You might be able to get to, say, a local supermarket and back but any longer journey risks seeing the temperature gauge climb at an alarming rate. (I'd still advise against even trying a short supermarket run. When my car had a dodgy head gasket I managed a few short trips but on the next journey I only got 2 miles from home, starting from cold, before the temperature gauge went off the scale).
I hate to think what a new head gasket for a Cadillac is going to cost you. I'd be surprised if you could get it done for under £1000. When my 1.6 litre Ford Zetec needed the job done, about 5 or 6 years ago, most garages wanted around £700 to £800 for the job (although I got it done for £487); I'd be prepared to bet that the same job on a North Star would cost far more!