>I'm running on i386. So I should just installboot?

Yep.

I haven't done LFS myself, but the i386 bootloader includes code to
read from an LFS filesystem. "installboot" plops the file on disk, and
writes the disk offsets of the bootloader code so that MBR-level code
(which is loaded by the BIOS) can read in the netbsd bootloader.

As long as your new LFS partition fits within an MBR slice which
can be read by your BIOS using INT13 calls, you should be fine.
(Most new hardware can read up to 128gbytes, but if your an controller
is older than about 18 months, or your disk is bigger than 2 gigs,
you should check).