Mike, don't under-estimate the experinces of those of us who lurk here.
I (in a prior life) worked for a boat building company that, for a short period of time, built the Rhodes 19 after O'Day stopped producing it. So I am familiar with the boat.
Installing a mast tabernacle is the same on pretty much any boat. What you need to do is get two mast base castings and a mast hinge, bolt a casting to each of the hinge plates, cut the mast off at the height ncessary to allow you to insert the hinge, and pop rivet the castings in the mast 'halves. If you can, build up a pad for the bottom part of the hinge to lie flat on the deck so that the hinge just barely touches the deck (it MUST remain being supported by the lower mast 'half') and bolt the hinge 'half' to the deck. That way the lower part of the mast will be permanently installed and will not come out when you step/unstep the mast (making it easier to step/unstep the mast).
The mast parts needed can probably be obtained from Dwyer Aluminum Mast Co. They have a web site at:
www.csiworld.com/dwyer/dwyer.htmIt has great information on extrusion dimensions and fittings, with costs.
The Rhodes 19 Class has a web site at:
www1.shore.net/~msicomm/R19/R19.HTM
Keith A. Bay (kabay-at-execpc.com)