Problems with booting & MBR

[vdm.fbsd at virgilio.it]

In the first disk ad0 (master) of a computer at office I had installed linux
to be used as a postgresql server. Later I installed FreeBSD 5.4 (just to
have a go at it)  on a partition of the second disk ad1 (slave) to be a postgrresql
server too.
Linux lilo was the boot loader, booting linux by default and freebsd.
Now, convinced of the high quality of the latter I eliminated linux from
the first disk unfortunately without modifying lilo, and reshuffled the FreeBSD
filesystem as in my latest /etc/fstab using also the first disk:

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/ad1s2b             none            swap    sw              0       0
/dev/ad1s2a             /               ufs     rw              1       1
/dev/ad0s1a             /usr/local      ufs     rw              0       0
/dev/ad0s1d             /var            ufs     rw              0       0
/dev/ad0s1e             /tmp            ufs     rw              0       0
/dev/acd0               /cdrom          cd9660  ro,noauto       0       0

Now, even though I read the booting procedure in the handbook and somewhere
else in the internet, issuing the various "fdisk -b B", "disklabel -b ..",
"boot0cfg ..", I was completely unable to modify the MBR and make this FreeBSD
only computer boot directly into this OS.

What should I do with the configuration I have?
A straightforward and step by step explanation is highly appreciated.

P.S. Is it possible that some problems can arise by the fact that I used
an "a" slice (ad0s1a)for mounting /usr/local and freebsd starts from ad1s1a?

Ciao
Vittorio