If you nre going to seam weld do the whole shell or don't bother. All it will do if you only do the engine bay is pass the stresses back through the rest of the shell that isn't seamed and put more pressure there.

And agreed - unless you are running stupid power (I would class 320 as 'stupid' power but not in any derogatory sense) and doing track days etc I wouldn't bother with it.