here's something we ran into when doing the swap. we took the 10 bolt drum rear out of an 83 z28 and put in a 9 bolt disc from an 87 gta. all went well until it came time to hook the steel lines to the main rubber line. as you can see in the pics. the 10 bolt lines do not have the non threaded extension on them. so when it came time to screw the 9 bolt hard lines to the 10 bolt rubber line, they wouldn't go in. they are the same size. just longer.

so you would think hey just switch the rubber lines. no deal. the 9 bolt rubber line is just a little bigger in diameter.

option #1 - cut the main hard line and use the screw there. only one cut and flare. problem about 1" away, the line makes a 180 bend. so not enough room to put the flare tool on.

option #2 - cut the hard lines to the calipers and use the 10 bolt ends on the 9 bolt lines. easier task because much more room to slide the screws back. this way you only cut the flared end off. so minimal line cut off. flare the ends and you are back in business.

now this may be because the car was an 83. may have changed in the later years. but heads up on this one.

middle ones are for the 9 bolt. outer ones are the 10 bolt ones.

 

The fittings are totally different, in more ways than just those "extensions". The 83 car uses SAE flares; 84-up cars (including all 9-bolts use metric "bubble" flares. IIRC the SAE thread will go partway up into the metric fitting before the threads start to bind; so it seems as if they're the same size; but they're not. You can see that fairly clearly in the pic: the metric ones have a slightly finer pitch.

I ran into the same thing when putting a 91 or 92 rear into my 83. I ended up using the late-model rubber line, and cutting the last 6" or so off of the car-side steel line, and splicing the metric fitting onto the end of the SAE line; not a totally trivial task, but manageable.