Getting from 5.1-RELEASE to RELENG_5_1

I am looking at moving various hosts from 4.x to 5.1 but have run into
a problem with my test machine.  I've successfully installed
5.1-RELEASE (from CD) but want to rebuild the system to customise it
to its environment.

The machine in question does not have enough local disk space to hold
both /usr/src and /usr/obj.  When I tried to NFS mount the space, the
'make buildworld' would consistently wedge the machine in "stage 1:
bootstrap tools" building games/fortune/strfile.  I tried using local
disk for /usr/obj and NFS mounting just /usr/src.  This got somewhat
further but again wedged the machine.  "Wedged" means no response via
network or local keyboard, needing reset to recover.

Has anyone else seen NFS problems with 5.1-RELEASE?  The client is a
P-133 with 96MB RAM.  The server is a PIII running roughly 4.6.  They
are both using 100baseTX full-duplex 802.1Q trunks to a common switch.

My alternative (preferred) option is to do a build world/kernel on
another faster machine (the NFS server used above).  This fails with
multiple definitions of _sigaction and _sigprocmask building
libpthread.  Looking back thru the archives, I gather this has been
fixed in -CURRENT but it's not in RELENG_5_1 because it's not
security-related.

Any suggestions for a way forward?

Peter