I'm not a body man, but I don't think you need one to see the issue you've got going on. If you're careful, you can probably make it better. It's also possible to make it worse.
Understand that shimming up or down the frame to tub (in the middle or rear) may effect the rear door gap from top to bottom. -Down in the center, or up in rear will close the gap at the top. It may also effect the way the door latches -or the other door.
Before you go messing with body shims, I'd suggest you look at the built in adjustments. The door is held by three points 1)the top hinge 2)the bottom hinge and 3) the latch. All three of these (if they are anything like a TR3 or TR6) have a little "wiggle" room built into them. Moving any of them may result in paint damage in the area of the attachment. It only takes a little hinge movement to align the face of the door with the face of the fender. Once that is aligned, you can try to adjust the door gap to fender to be straight while minding the door to sill gap, and while minding the rear latch and rear positioning. Then you can see how well it latches (or not), and try shims to get the rear edge of the door at the proper height or latching well, or in alignment with the rear fender.
It can be very time consuming, because one adjustment will almost always effect another. The hinges are the easiest to adjust. After hours of adjustments, you may find the front fender is not aligned with the rear, so no matter what you do with the door, something will not be perfect. Or you might make a little tweak, and see how perfect you made it. Also, it's best to start with the weight of the car supported by it's wheels with tires properly inflated. These frames bend a little when jacked, so you want to make adjustments that will hold true when you set it back down.
If that's not enough, the front fender also has a little wiggle room. If all the bolts are loose, you might be able to 'flatten' it out along the door edge/slightly change the profile, or move it farther inward to better match the door. If I'm not mistaken, the door must be removed for access to the fender bolts. Masking tape and markers can help save time by marking where it was or needs to be.
When I did my TR3 body work, I spent more time on the door gaps than anything else -except sanding.
Good luck, take your time and it will get better.