anyways, i was going to hook it up with a 30 amp relay and a inline 30 amp fuse for each, 1 for the high speed and one for the low speed. with an on-off-on switch.
The Mark VIII pulls over 100 amps for milliseconds during ramp up. That little relay will shoot poo all over itself in a short amount of time (and inductance will wipe out the Normally Open contact anyhow).
For the easy way to do it (and a real nice way in hot Texas), use a DcControl unit. Other guys (Urban for one) have had good luck with the SPAL controller. FAL also makes a decent controller but I'm not sure it can handle the Mark VIII.
If you really want to do this using your own relays, At a minimum, I would get a Bosch 75 amp relay for high, and use the factory input to the HEDF relay for your new fan. Even better is to get two 75 amp relays and do this for both low and high. The will be controlled like stock but use HD relays instead. This will last a lot longer. Of course, by the time you buy two 75 amp relays, you're well on the way to a quality controller (though with the SPAL and maybe FAL, you still need a high speed relay IIRC. With DcControl, you do not).
No matter what relay you use, the flyback diode is a good idea.
Actually, the easiest way is to use a Dc Controller. It's along the lines of a 5 wire hook-up. Because there are no relays, it makes the wiring really easy.
Two 75 amp relays will work fine. You really would probably want a temperature sensor of some sort unless you like flipping fan switches non-stop.
Knowing exactly what you demand of the installation would help us to make suggestions. For example, a Dc Controller on a 1320 car (only) would be a waste. But on a street car, using mechanical switches with no controller could be a disaster.
It's really up to you then. The two 75's would be robust. You could try to save a few bucks and use a 50 amp Bosch relay (they cost like 6 bucks instead of 20-25) for the low speed.
As I mentioned, figure out all your parts costs before ordering and compare that to using a Dc Controller. I did this with a friend whom was going to use relays. With his chosen controller, etc, it was going to be 92 bucks. It was a no brainer to just use the Dc Controller instead.