I suggest chasing the problem systematically starting with cheapest/easiest fixes first. Disclaimer: I'm not an engineer, but I have slept at a Holiday Inn before. Did you try pulling the Radiator Shroud off? If it vibrates with the shroud on, but not when you take it off, that will eliminate you chasing a suspension problem that possibly doesn't exist. You didn't mention if a hood and hood sides are part of the future plan. It might be a case of you not needing the fan shroud without the hood, but you do need the shroud with a hood and hood sides. 1. Could be the Shroud blocking air at high speed and not letting enough flow through the radiator: Try taking the shroud off and the vibration stops at high speeds, then you'll know its the shroud not letting enough air to flow through the radiator. Drive it around for a while without the shroud and see if that stops the high speed radiator vibration. Drive it around for a while without the shroud and see if that stops the high speed radiator vibration. If that doesn't change it, then the shroud isn't the problem and it could by the lower radiator mounts or something else, so put the shroud back on and proceed to the next step. 2. Could be the lower radiator mounts: If it doesn't change the vibration at high speed after fan shroud removal, put the shroud back on. Then the problem is something else, possibly the lower radiator mounts, suspension issues, or tire related. Put the shroud back on and address the lower radiator mounts next as that is the next easiest and cheapest to address. 3. Could be Shroud blocking air, but you actually need the shroud: If it runs too hot w/o the shroud at low speed and idle, then you need the shroud and you need to fix the shroud so it isn't causing the vibration. Possibly putting a mail slot with a flap cover on a hinge like you stated earlier. 4. Could be the front tires: If fan shroud removal doesn't change it, you put the shroud back on, and lower radiator mounts don't fix it, check you tires for balance and making sure they are round. Ive read posts about tires needing shaved because they are out-of-round. 5. Could be that you have too much run-out on your rims and it doesn't show up until you reach high speeds: If the above steps don't correct it, then next step would be to check the wheels (w/o the tires on them) for run-out. They could be a little off true, and need fixed. I seen a video of a wheel repair shop using a press to true up bent wheels. 6. Could be that you need to adjust your front end alignment: Take the car to an alignment shop that can align an older car. 7. Bonus round. You plan to run a hood and hood sides: If you never plan to run a hood and hood sides, don't read this step as it will be irrelevant... However, If you plan to run a hood and hood sides, then once you add the hood and hood sides, you might need to put the shroud back on to pull the air through the radiator. With the hood and hood sides on the car, the engine bay will be a warmer area, but it will change how the air flow's around the radiator and you'll eliminate the turbulent eddys of the air flow curling around the back edge of the radiator, thus possibly stopping the vibration. Think of the videos you've seen of cars in a wind tunnel. At the back of the car, the air curls around making a low pressure area. That's why a lot of the super cars use the rear lower bumper air diffuser thingy. If you put a hood and hood sides on, you'll eliminate that air flow turbulence, but you'll change the air flow through the engine bay area. Hope this helps. Let us know some feedback with what you come up with so we can learn too. Don't keep that sweet-sweet knowledge to yourself once you find the solution.
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