If you are going to do two classes, I'd recommend something like this.
1) Spec OPC Tunnel - rules like Jerry described above... This class will have the biggest bang for the buck and is a lot of fun and very competitive. Hulls would range from 26-34", with TS-2, VS1, or any other hull up to about 30" or so... NAMBA FE "P" Power specs, with 4S1P being the best choice... You'll want at 4000-5000 mah pack sizes to make 6-laps on the 1/6th mile safely, which is how we ran it at the 2008 FE Nats... LOADS of fun...
2) P-OPC Tunnel - In an more "open" format, 4S2P sized lipos will be needed... but will provide MORE than enough power to propel a 34" tunnel to well over 50mph... Full blown "P" power would actually fair better with .45 or 7.5 sized hulls, but we've done it in 30" hulls and they are still very raceable... The TS-2s start getting crazy at these speeds, but any of your 3.5 mod sized tunnels would be up to the task if they'll hold the cells... there are all kinds of motor choices out there for a class like this. It's going to be about $300-$400 more to build a competitive boat... maybe even a tad more depending on how you go about it... but they will blow your minds as to how fast they go when setup properely... You could go even bigger and use more battery, like maybe powering it with "Q" specs, which is 6S Lipos... You might be able to get away without paralleling these if you choose motors correctly, which would be purchasing two less cells, but the motor and controller would have to be higher voltage capable, so this might be a wash... I'd want at least a 36" or even larger hull to handle this kind of power...
Like I said previously, I think 4S is as big as one needs to go for battery power. It fits into the hull, and can easily be configured with FE power to race in the mid to upper 50s... quite easily actually...
The spec class, however, is the best bang for your buck, and I think you'll be impressed with the performance of these boats, even with the cheap motors and ESCs... A setup like this can compete quite handily against all but the very fastest 3.5s from what we've experienced... they are basically ALWAYS "on the pipe"!
Here's a video clip from our last race weekend... The first place boat (White w/ Black) is Brian Buaas with a TS-3, the second place boat (RED) is Greg Schweers with a TS-2, and the third place boat there (White) is Lohring Miller with an older LeeCraft hull (I think that's what it was)... I was out, having flipped my WoodStuff 28 in the two previous heats and deciding that I'd like to get some video to share with all of you instead!