Is the belly pan still in place? Again, I'm assuming it is. If it's elevator bolts "out in the field" of the plywood, you can set your skill saw for a 5/8" deep cut and cut out a small square around the bolt head, break up the remaining little plywood square and then use a bolt cutter to snip it off or a cut off wheel . . . If it's bolts out at the perimeter in the bottom wall track, thru the plywood and into the outriggers, AND everything (belly pan, outer skin) is still attached then you may need to take a "happy pill" and keep working on it. You might try a reciprocating saw and try to be as parallel to the side as possible (so you don't run the blade thru the side of the ext. skin) . . .
Aluminumbskull with Led Balloon in Drag
Birch Plywood and Aluminum go together like
Peanut Butter and Chocolate