Are you using the same temp controller settings as before? Think it's like 600f off/ 400f on when your timer is activated along with 275 minimum flue...I've got the same chimney height and a couple times every heating season when I get high enough winds from the south (no tree protection from that direction) I will see 500F+ flue temps. Otherwise 350-400 is normal for me as well. During those relatively rare events I remove the paper clip that holds the air inlet slightly open and use aluminum tape to reduce the openings for secondary air by about half to tame things down a bit. I did purchase a BD and probably should install it but so far have not. The extra flue temp during these events also increases plenum temps and with no tree protection from the south the house needs the extra heat.
Also do you still own both of the original Tundra1 no cracks?