I would simply use the MBR for the 60GB variant and adjust the data partition afterwards. I've done similar things (converting various CF cards to an Ipod Mini, even with cards that were notably smaller in size than the Ipod drive) and it worked fine so doing so should be safe. Reformatting will loose any data on your Ipod anyway, so if anything goes wrong you can always restore the Ipod using Itunes (which will, on Mac, obviously use HFS+, but afterwards you'll at least have a working Ipod again). So:
- install the MBR for the same Ipod with a different size
- run an fdisk tool (I've done it on linux, not sure which is the appropriate tool on mac). Delete the data partition (the 2nd one) and recreate it with the correct size. Make sure it doesn't overlap with the firmware partition (the 1st partition with type "empty").
- format the data partition and restore the firmware partition as described in the page you've linked.