Hi. I had the same issue with a 73 I did years ago. No front clip, just a rad and hoses. Fresh 350 running for the first time after rebuild. Checking timing, and oil pressure, etc... then poof, rad cap blew open. In the end, having the fan shroud on would have saved the boil over. Without it, the fan doesn't even draw air through the rad. Hope this helps, and best of luck on the ride.
Jim.