Ah yes, been there done that. Well actually just getting over that issue. The lack of the heater core has probably forced you to route the heater hose in a loop from the manifold right back to the bottom of the rad, which may be moving the coolant too quickly for it to cool. I also lack a heater core and have had overheating issues with this setup. I flushed my original rad until i was blue in the face, but the temp drop was not significant ( maybe 5 degrees at a time). I had my rad hot tanked at a local cooling system shop (30 bucks), and pulled the water pump while I had the rad out, with the pump removed I ran a light degreser through the block for about 30 mins. When I bolted everything back up, I noticed a huge temp drop, I can now actually cruise during the day. Bottom line is over the counter rad flush kits dont get rid of 40+ years of crap that easily.
Also, when I got my rad back from the hot tank I noticed what looks to be cotton? floating around in the upper tank...Does anybody know what this is ( maybe a crude filtering media)???