I had 0 issues with the Champion for leakage, it didn't cool as well as I would have liked. Olds did every cubic inch displacement increase on the SBO by increasing the bore, so the 260 and 307 have thick cylinder walls and very few issues. The early 350's also have thicker cylinder walls and cool quite well. The 77 and later 350 and 403 have much thinner walls and the 403 has siamese walls between cylinders like a sbc 400 and an even bigger 4.351" bore. Basically the 403 for reasons mentioned and the Olds 455 with big heavy, high friction bearings and fairly thin cylinders are the majority of Olds cooling issues. I have had few issues with the 307. The only times one overheated on me when a water pump bearings exploded and another leaky water pump was drove a bunch of miles by my wife. The recored 50 year old rad, a other brand 2 core rad that cooled better than the Champion had issues. That 2 core now barely cools my 260 thanks to a ton of stop leak, leaking between the fins. I also had an aluminum plastic rad split and those stupid chrome hoses leak multiple places. If you went through as much Antifreeze as I have in the last, you would try a 7 pound cap. Don't get me started on transmission leaks, stop leak and THF fluid has helped a bunch. I should have bought stock in both Antifreeze and ATF. My point is, the 13 pound cap will work fine and put less stress on hoses and any seems for leaks. The Champion will be more than enough for your 307, which is a great reliable motor in my experience. I am planning on a .039" overbore on my 403 and a 3.5" stroke, 424 ci. I plan on using the huge nearly 35" wide with dual 1.25" cores Cold Case rad in my 70 Cutlass S for cooling. Having better airflow and a much wider rad, it should cool with ease.
