you need to be checking the air fuel ratio when driving at the speeds that the heat build up is occurring. really this can only be done with a wideband o2 sensor or on a dyno. all you can effectively adjust with vacuum is the idle mixture.
Couple of things I would add as food for thought. Shrouds.. or even fans for that matter., are not needed to cool an engine traveling down the road at highway speeds. Look and listen to all the newer cars and trucks with e-fans to get an idea. If you're still heating up during that kind of airflow level?.. then I believe Bryant has you looking in the right direction with his tuning advice. Also.. running high temps around 220-230 is ok for improving emissions readings and computer controlled cars that can actually correct things on the fly(via coolant and intake air temp sensors) to change fuel and timing maps to avoid damage resulting from such thin detonation margins. But it certainly won't make more power as the computer "pulls away the tune" to better protect itself and compensate for the higher temp air/fuel charges reduced density levels. The cooler the charge.. the greater the detonation prevention becomes.. which then allows us power mongers to sneak up on additional power output with a more aggressive tune. IF.. we're so inclined to take advantage of the opportunity. I typically run 160 t-stat's in the summer and 180 in the spring and fall(even with EFI stuff) and change my tune to match the inlet temps. When I can actually keep my foot out of the additional power gained.. economy improves as well. Oh.. and if you have a thermostatically controlled fan relay?. the heat soak will never even come close to 250 because they will kick on when heat soak sets in and back off when it's under control. Set it to about 20-30 degrees over the t-stat's rating for best results.
