I have a 69 L78 that was rebuilt last fall. I have run it a few times in the garage and thought it was running a little warm just past the half-way line on the temp gauge. Today I took it for a drive and in about 2-3 miles it was running a line away from "H". I came home and kept the rpm's down and it ran at that mark or one less. After setting in the garage for about 5 minutes the lower radiator hose that connects to the bottom of the water pump blew off. Any ideas where to start? Nothing came out of the overflow by the radiator cap.

Providing your system is in good condition, about all that can cause over heating is a leaned out carb and/or retarded timing. Ours works fine.

Could also be a bad thermostat or the fan clutch.

did the thermostat open?
was all the air purged out of the system?
does the radiator core flow like it should?
are the cooling fins in the radiator blocked and/or bent?
what kind of fan are you running?
does the water pump work properly?
are you running the stock pulleys or an aftermarket underdriven setup?
where is the timing set?