[justpassntime]

I like Tango Charlie and Sayyad's thought on this.

I think though if you used enough aluminum tubing to help support an aluminum belly pan tray of sorts. The aluminum itself would be enough for the heat sink without the need for fins. It could even be ran through power steering or tranny coolers if needed. Doing it this way you could probably do away with the radiator entirely.

You may need a small overflow tank to hold additional coolant so your cooling system still retains the same amount of fluid.

Another thought is aircraft hydraulic system filters. Some were large enough to hold at least a quart of fluid and had heat sink fins built in.

If you wanted to retain a/c though you would still need the opening.