An adapter to fit the 2 ROMs should be pretty easy, all pins wired straight except /OE which needs a bit of logic to decode which ROM can output to the data bus (something like /ROM0_OE = /OE & !A1 , /ROM1_OE = /OE & A1)
But it's the content of the ROMs thats gonna cause you problems, IIRC the A1200 ROMs were compiled for an '020 CPU, so there's a good chance the 68000 of the A600 will run into instructions it can't understand and you'll be in guru hell

(Just remembered, utility.library is 020+ on the A1200 for a couple of functions too...)
Then there's the odd differences between the A600 and A1200, eg scsi.device in an A1200 ROM will only run if it finds a AA Gayle, so you won't be able to use a HD on the A600
Like C64_d0c said, it'll be a lot easier to buy a 40.63 ROM.