[Randy in ca]

Assuming that your body is already setup with the correct rumble seat body components in the lower body opening area (i.e. it is a rumble seat body), I think the only concerns with using a 1928-29 lid on a 1930-31 body are the locations of the handle/latch on the lid itself and the location of the striker on the drip rail. 1928-29 had the striker mounted on the front edge of the driprail, while 1930-31 used the rear edge to compensate for the change in the location of the handle/latch (which was apparently done to avoid body damage if the lid was closed with the latch in the wrong position).

I have only done the opposite (1930-31 lid on a 28-29 body) not the other way around. This involved only relocating the striker from the front to rear edge of the drip rail. In your case, I think all that needs to be done is to move the striker from the rear to front lip and recess the rear lip for latch clearance similarly to a 1928-29 driprail. Clear as mud?? Here's some pictures (Roadsters) I saved when getting this figured out. If you want to be totally authentic, look for a 30-31 lid. Good luck.