You are nearly there but the hard disk needs at least evox on it to be able to FTP to it (in the normal sense I mean, not telnet etc)
You say you have a chip in your XBOX, what chip is it and what bios? Or do you actually have a softmod done on the hard disk? If you have a softmod then you cannot use a slayers disk to put in a new hard disk, if you have a chip with sensible bios on it then you can easily...
If you have a chip then FTP the contents of your old hard disk (C and E drive if you like it the way it is and F if you have anything on it) or just the UDATA TDATA folders if you only want your old game saves....
Then you just boot off the slayers disk (if you've burnt it right!) with the new hard disk with jumpers on it set to Cable Select position.
If you want it like your old hard disk and you backed it all up: ....I'd choose restore retail hard disk option, it will format the C and E and set it up as a standard hard disk, then manually format the F drive using the Slayers disk still, and then finally just install evox on C (or wherever you had before) remove disk reboot...
Evox will come up missing lots of stuff, go the system management and setup your IP address / gateway for your FTP to work again, reboot, the FTP into the XBOX and copy your C and E drive backups over. You now have a 40GB (or whatever size you said it was) hard disk that is the same as your old one but with a bigger F drive. Make a folder in F called Games to hold your games (make sure your evox.ini looks in that folder to auto add games to the games list).
If you don't care too much about your old hard disk then just choose a new full build (retail hard disk) and then manually format the F drive after. Or do it any old way you fancy really

Cheers,
PimpleX