@redrumloa
If the project really is that critical, then replacing all the Arcnet cable with thin ethernet coax is gunna get to be more costly than replacing it with Cat5 AND supplying ethernet cards to all the machines.
One other advantage is that the Cat5 can be run to a central switch (or hub if you prefer) without all the hassles of a peer 2 peer network.
Of course, if it's Amigas involved, then the price of network cards negates all of the above.