I have a 409 in my 60 GMC and it is not quite there yet as far as coolness. (Actually it's really cool but....)
I have a new 4 core radiator and a steel fan shroud similar to the 409 cars of the early 60's Th rad. is a new but vintage design, not the crossflow type sold by some vendors(which require hacking to install.) I have a stock fan with no electric fan. It is for an auto trans so it has a trans cooler as well. I am running a 195 deg t-stat. On Sunday it was about 60 deg. for it's maiden shakedown run. It sat at 195 for 70 miles until stop and go traffic caused it to go up to 210 deg.
It didn't boil over, but it was noticeable. On the way home, it rode at about 200 and was fine at highway speeds. no further traffic was encountered.
I think I still have some air in the system and will bleed that out. I also want to put in a surge tank to keep things clean.
A big block is going to run hotter, so do whatever you can to help it out. I don't think that stock 6 cylinder stuff will get the job done.