75#'s of Eva (or most other rock) in a 40 with a DSB isn't going to leave much room, unless you intend to just pick out your favorite pieces.
Having an extra power head or two on hand is always a good thing. You *would* want them in the buckets/tubs/cans for curing the rock. I think your best bet is to make sure to have the supplies ready for the rock before you buy it. Including enough tubs/buckets/cans, an extra power head/pump or two, at least one full spare large bucket of salt, and misc. other items you may want or need, such as new and sanitized - nylon bristle scrub brush or two, needle nose pliers/forceps, thick rubber gloves, water changing hose, etc.
You would also be well prepared to have the ability to do at least a 50%+ water change daily if needed. My skimmer has been pulling out about 1 gallon of wet skimmate an hour when I let it. That water needs replaced as well as the water for cleaning cycles, detritus removal, and water changes.
The 75#'s of rock for a Manderin thing is along the lines of saying 1-1.5lbs/ gallon for rock, xWatts/gallon, etc. Too many variables to say with much certainty. Ever hear of the Tang police?

Granted, the more rock the more likely it will be able to provide enough natural food for your Manderin, the right amount of light capacity for what your keeping is important, and the more water for Tangs the better, but absolutes are few and far between in this hobby.
The better you take care of whatever rock you get the better it will be able to take care of itself and your livestock.
With the Manderins, making sure the tank is stable (under relitive control), mature (Many say at least a year old), and not too clean (what do the pods that Manderins eat need?) seems to be at least part of the key to their succesful longer term keeping. Finding ones that eat hand outs is also a plus.