Hi James -
There are several points you need to keep in mind when determining which aeration system to use for this 40 acre lake.
Be aware of maximum PSI on the rotary vane compressors. Most can't run over 10 or 15 PSI which can be very limiting when it comes to long tubing runs, deep diffuser placements and higher altitudes. Instead you should look for a system powered by piston compressors. They have a max psi of 20 or if needed 35. Pistons also are easier to maintain than the rotary vanes. With the right type of diffusers you will have no problem running tubing thousands of feet.
Which brings me to the airstones. Please look into membrane type diffusers. These are self cleaning and much more efficient. Airstones will clog over time and require reqular acid baths. Membrane diffusers are self cleaning and do not ever need to be removed from the bottom of the pond.
You should make sure that you are properly aerating this pond. A system that does not have enough 'lift' will only result in mixing the bad bottom water with the good water with out ever pushing the bottom water all the way to the surface where it can expel built up gases and absorb oxygen. This will result in lowered O2 through out the water column and increased CO2 which is a very important nutrient for algae.
Look into Vertex aeration systems. They can assist you with accurately sizing this lake and determining how many diffusers you need and where they should be located to prevent future winter fish kills.
Do a search on this site to read more information on aeration systems and preventing winter fish kill.