Can you snap a picture of your Tstat housing to Filler to Radiator setup and post? Picture might get you further down the road to a solution more quickly. The filler has an important dimension between the top mating surface and the bottom pressure relief seal surface. If it aint right, you could be sucking in air while you're driving instead of your reserve coolant at rest. There's been some sound advice given here so tell us what you've done so far. Small blocks and Windsors are notorious for needing a good burp. Does your temp spike and go back to Tstat temp a few times up and down during the first few miles or does it steadily rise and stay there no matter?
At only 6.5 miles, your gonna run a little hot during break-in as the engine is working harder than it ever will when mating surfaces get acquainted. What
oil are using for break-in?