These monsters date back to the time that I was doing a lot of garage work on a part time basis. First of all, the 'Y' block was not a hot running engine with a good cooling system. But, the main source of problem that I, personally invariably found was someone installed the thermostat with the pellet facing out towards the radiator. Also, you do need a good 180* thermostat. Most of the new ones seem to be 190*/200* for newer cars. A 160* will run nicely, but will also cause sludge build up.

(Then, there was that '57 Ford that I owned with a 312 - that @#$% car would just for spite would blow the anti freeze out - and that's with new hoses, thermostat and radiator cap - then it wouldn't do it again for months)

PS: Make sure you have the correct thermostat that fits the machined groove in the housing and don't crack that housing either - it's easy to crack and my guess, replacements are hard to find these days