Mike, I would suggest you think of the decisions you will be making re: spreader design to be those of a sub-system. You don't say but I assume (there I go...) your spars are wood. You also don't tell us if you are working from the original hardware
for the spreader bases. Were it mine to do, I'd start with the spar cross-section and construction and build up my decisions based on the hardware
choices available (if you must purchase
them). It could also be that you have the original hardware but it does not provide for the raised angle that Kai Nui mentions; it was pretty common in the 70's and 80's to find spreaders placed perpendicular to the spar, in which case the design of the bases might be simplified...but that doesn't mean it's a good idea now. There are numerous end tip designs, as well.
Given your 'project', how good a reference library have you been able to assemble? This might be a good time to visit Amazon, shop among the resellers of used books
there (or any other good source) and pick up a few 'old' books
that, for these kinds of issues, will not be dated.
Ross Norgrove's Rigs and Rigging
was a standard of its time (15? years ago)
Don Street's Ocean Sailing Yacht I and II get into many basic structure and design issues, and Vol. II was illustrated by Bruce Bingham, which makes it especially useful
Shipshape & Bristol Fashion has lots of rigging
and other techniques for finishing off the yacht
Dan Spurr's book on refurbishing an older boat (sorry, title escapes me) was also illustrated by Bruce Bingham, another good reference.
Sailor's Sketchbook by Bruce is a compilation of projects, well illustrated, that the owner can do; they were published for some years in Sail magazine. I believe a later volume comes from the CW column Bruce subsequently did.
With the exception of that last pub, I wouldn't think any of these books would cost more than a few dollars. Supplement it with Calder's Boatowners Electrical
& Mechicanical Guide, and those references
will serve you will, even provide alternatives to what you hear from Web sources.