Well I'm certanly not the bootloader expert around here, but I think by your using an external 8Mhz crystal the only changes you would have to make are:
mega.upload.speed=57600 to mega.upload.speed=28800
and
mega.build.f_cpu=16000000L to mega.build.f_cpu=8000000L
Let us know how it turns out.
Lefty