[Mark Yacavone]

David, It really depends on what kind of finish and look you are trying to achieve.
As Larry said, you'd probably want to fill and grind from the engine side.
For a quickie, you could make one patch panel that would cover the heater hole and surrounding small holes, all in one shot. Pop rivet it on from the front. A couple of 1/4 -20 bolts and flat washers for the remaining holes , a can of Krylon semi, and you could be done in an hour. All up to you.