Cale,
The last thing you want to do is potentially elongate those holes by grinding or flapper wheeling.
You can probably get them in by heating the adapters to 300 F (no more) and chucking your studs in dry ice or liquid nitrogen. Handling is a bit of a pain, but it can be done. .021\" in not too much of an interference fit to cause any shouldering problems or cracking, unless you adapters are cast and full of porosity.
Csaba