For the front a simple triangulated strut brace is sufficient. Or you can weld a piece of tubing across to connect the two shock towers. If you get crazy you can install an X brace to connect more of the front chassis.
For the rear I would suggest connecting the two shock towers with a piece of tubing and also weld the tubing at the midpoint where it comes very close to the rear cross member. In addition to that, I would add some triangular gussets to the outer sides of the shock towers or tubes that protrude into the engine compartment.
Is it all necessary? I don't 100% know for sure, really depends upon spring rates. If you can weld nicely and want to tackle the job then it certainly will not hurt.
Fernando's web write up is a nice reference to check out:
http://www.readynet.com/myproject/index.html