Did you fill the coolant from the crossover plug? If not then that's your problem. The crossover plug is the highest point in the coolant system and air pockets will get trapped if you do not burp it from there properly.
In the early years of tackling engine coolant temps...
drilling the stock t-stat was what many of us were doing. pros and cons with doing that. But its not ideal anymore as we have a great 170F t-stat that was birthed by a member on this site. The 170F t-stat made by this member hasn't disappointed.