The first problem is that the O'DAY 27 weighs in at almost 7000# compared to about 4000# for hte O'DAY 25 Keel/CB model (NOT a "swing-keel", O'Day NEVER built ANY swing-keel models!!!). Draft for the 25 K/Cb is 2'3" to the 4' or 5' draft of the 27 (depends on if this boat has the optional deeper keel or not).
If you really want to build a trailer for the 27, I'd suggest getting a trailer frame built to take the weight of the 27 and either purchase proper boat stands (to be welded in place on hte trailer frame) or plan to build a wooden cradle on a flatbed trailer. To get everything properly setup, I'd haul the boat out on a travel-lift or crane then set the boat on hte trailer and get everything properly adjusted. Then next year you can start hauling/launching off the trailer. Better find a real deep ramp and have a good heavy-duty truck to do this.
Best idea, and probably less cost than building a trailer now, woulf be to hire a local boat hauler with a hydraulic trailer to haul the boat out this year and bring the boat to your yard, you then have all winter to build a good solid trailer to use next Fall to haul the boat out.
If you want to see what happens if you don't get the trailer right for an O'DAY 27..... check out the video on YouTube of someone launching their 27 and breaking the trailer in the process (I don't know..... maybe it was planned that way.......?).
I've included some pictures of a couple of 1978 O'DAY 27s, no scale drawing, but it still may give you some ideas of what you are dealing with.
The You-Tube video is called "Launching my 27 O'DAY", I couldn't find a way to provide a link. He appears to be using a Dodge Durango SUV.... and the vertical supports on hte trailer appear to fail just as the boat hits the water, I don't think he had any bracing to strengthen the posts and prevent them from flexing and that looks scary! The basic frame looks a lot bigger/stronger than a typical trailer for a K/CB 25 too!