conversion
I have done something like this before more then few times. My Dad and I had a couple of customers that would buy rust free coupes and rusty convertibles and we'd convert them. That was a little different then what's going on here, but I've put sh!t together a couple of times that looks like it got hit by a train. Some cars are worth it.
I've read a little of what you plan to do and I have to say. Try to use as much of the Sunbeam as possible. I'm not saying use the interior bits or running gear, but the skins and inner panels wherever possible.
Here's how I would go about it. I would check to see how the Miata will fare when you cut the top side off. If It looks like it might give way a bit I would brace it up, so it doesn't flex. I'm not sure if it has a frame or not, but if it does. You'll be aright.
As strong as a Sunbeam is it will need bracing up. This should give you some idea of how stiff your end product has to be, so the doors don't bang against the quarters. You can always put a rubber snubber between the door and quarter. They use to do that back in the day. The rockers are pretty hard core on the SB and will keep the bottom gap of the doors. The top is another story. You should put X like bracing in the place of the doors. You'll also need to triangulate the passenger compartment. Run a piece of tubing from the top of the back of one door pillar to the lower corner of the other front door pillar and vise versa. Tie them across to keep her from spreading apart. That is 6 pieces just for the passenger compartment and tack them where they intersect. If you haven't figured it out yet. I would do it in one piece. Your gaps will be easy to maintain and the body wont be twisted if you do it this way. When you go to cut the front end off the SB the hood gap is going to get out of control unless you tack some bracing inside the engine bay and tack the hood in place. I can still remember watching my hood gaps tighten as I was cutting the tops of my fenders. That was fun fixing that.. I would expect nothing less from the deck lid. I'd X brace that sucker too and tack the lid in place. If you must keep the hood and deck off. I would X brace both and run two across each opening. Kind like this "IXI" if you were looking down on it. 3/4" conduit works great for bracing. Just smash the end with a hammer, so it's easier to weld up. Just so we're on the same page, that's 18 pieces I would start with.
It's hard for me to explain stuff to people in writing, but that's about it.