66 Burb cooling system questions

[1966burb]

I am finally back on the BURB project. I purchased it a couple years and it came with a 350 engine with unknown history. I changed the water pump, thermostat, all the hoses. Had the radiator boiled out. ( the guy said he has never seen such a nice clean 66 rad) On one of my first short trips out I came back and as I pulled into my shop the top radiator hose blew off. Thinking I maybe had compression in the cooling system I took it to a shop and they did the sniff test in the rad. Everything appeared good. Now, when I drive it even for a short distance the top radiator hose gets rock hard and I am afraid it may blow off. I have taken some 3 hour trips towing my small trailer and had to problems, but I don't trust it. Any ideas??? About ready to rebuild the original 327 and get that put back in.