with your last statement would Probly mean that you do have a bad thermostat or the block does have a build up of crud on the outside of the sleeves internally. I personally would remove the thermostat and try it that way if it indeed has a thermostat. If it still gets hot without a thermostat I would then remove the new radiator and fill the block and head with a rust dissolver and let it set for a while and then flush with high volume clear water. A lot of water. Normally the only way you can get the B type engine hot is to run it out of water if you have a good radiator with good air flow and good thermostat.
I have a tendency of jumping the horse power, weighting them down, over tiring them and working them hard. Over heating has not been a problem.
When I would up the horse power I did clean the inside of the block. I was pulling the 12 foot spring tooth in third gear at the time I stopped to take the picture. That engine has a 4 inch stroke along with a few other mods.