What has worked for me was to reverse flush the block, remove the lower hose and upper hose, stick the garden hose in the outlet, the top angled casting with a rag wrapped it around to prevent leakage. Use full pressure, you can not hurt anything, this will blow out most rust debris.
Reverse flush the radiator from the bottom to the top, do not use full pressure or you will damage your new possession.
When you are done reinstall the hoses, buy 3 gallons of white vinegar and fill your system, do not over fill, you do not want vinegar on your paint so carry a damp rag with you while you are driving.
Test your hoses for leaks, then take your car out for a drive and build up some heat, you may have solved your over heating problem or not at this stage.
Leave the vinegar in for a week and drive it as often as you can, watch your temp. you really need an infrared temp meter to monitor things accurately, borrow or buy one.
Drain it all out after a week do the flush thing again, add 50-50 antifreeze and road test again, hopefully you will have solved your over heating problem.
I have had great success by adding a thermostat, we had one of our cars out today in 90+ deg. weather with a 160 thermostat and it ran all day long at 150 deg.
I don't think you said you had a dash mounted gauge, if you ever do don't trust it till you verify it with a infrared temp meter, one of my cars has a gauge that reads 45 degs. high. No I didn't check before I did all of the above.
Let us know when you succeed and what you did to whip it!