Just going to throw this out there, but maybe you've got an air bubble trapped in the cooling system somewhere? I don't really have any experience with big block stuff, but I know some engines can be a real pain in the ass to get the air out when you refill the cooling system. The funnel shaped tool that someone posted earlier is a good tool, I use that one all the time. Turn your heater on, so you know you have coolant flow through the heater core too, then get the engine up to operating temps by holding it at maybe 2500 RPM or so, until the fan starts cycling. Then let it idle, top up the coolant in the funnel if necessary, and then blip the throttle a few times, right up to like 4000 RPM, and let it idle for a minute. You'll see some air bubbles working their way up to the radiator and out through the funnel. The idea of blipping the throttle is that it is pushing the coolant through the system each time you hit the throttle, hopefully forcing any air bubbles out of lower flow areas. Keep topping up the coolant as necessary. Keep that up until you don't have any more air bubbles coming out of the radiator. Then when you are sure you've got all the air out, shut the engine off, take the funnel off and install your rad cap.
On the other hand, I'd think if you were hitting 235F easily just by driving normally, it would be pretty easy to make it overheat and boil. Most electric fans are coming on around 220F or so on cars I've worked on. As it has already been mentioned, maybe your temperature sensor reading is just being taken at a hot spot in the cylinder head (which will naturally be the hottest area of the cooling system), and it's not really a great indicator of true engine coolant temperatures. I am curious to know, when you find out, what the problem was.