The intake casting number for the 261 is 3836922 and the exhaust is 3835587 based on the '57 261 that I have in the garage. These are the same as what is on the 235 in my '57 truck.
I don't know if what you have will fit or not.
Out of curiosity, I did a quick search on your casting numbers and didn't come up with anything. Rather than try to adapt them, you might try to resell them to recoup your money. They don't appear to be common casting numbers, so they might be rare, or at least hard to find, for someone who needs them. The correct ones that I listed above aren't that hard to find.
Matt
