I agree with Kracker... Drill from the top, stepping the drill bit size larger and larger as you go.
As an aside, you made a comment about using dissimilar metals (like alum and ss), and you're right, up to a point. An Anti-seize compound MUST be used on any Stainless Steel nuts, bolts, clamps or other hardware to prevent galling and thread seizure. So, you can use them, if you put an anti-seize compound (like Never seez) barrier coat on them first. I've sucessfully removed nuts and bolts that were on a boat for 20+ years that were coated with an anti seize compound before they were assembled with very little trouble.
I've heard that it was invented for the military. Truthfully, I don't know who invented never seez, but coating all bolts before assembly was just about a required proceedure when I was in the Navy.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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