Re: transfer case & rear end leak
That's strange. All my dana 20 transfercase plugs miss the crossmember by 1 1/2". The '67 with the TH400 is about 3" away (because of the extra length I guess). I would check to see if the crossmember has been moved or butchered at some point, or if the drivetrain has shifted forward due to bad mounts, not likely but just a thought, it may be the reason for the brake line interference also. But again, I don't think it's it because you would have trouble shifting the transfer case.
You can do the seals without taking off the case (at least on a quadratrac), but it's much easier to do it with the case on the bench, unless you have good access. You'll need to pull the yokes with a puller. A vibration damper puller works fine and you'll have to find a way to keep the yokes from turning. I just used a pipe clamp. Get a seal puller or slidehammer with a puller attachment and pull the seal, careful not to nick the splined shaft. When you put the new seal in, a thin coating of permatex 2 on the outside of the seal housing will help stop leaks between the seal and the case. Don't bother with a seal driver tool. It won't work anyway if the splined shafts are still there. I have better luck with a block of wood and a hammer. Tap evenly around the seal. Once it's started it should go in ok. Just keep it straight. If it kinks, it's ruined.
Look at the seal surfaces on the yokes. If they're rough, polish them with 400 grit crocus or emery cloth. I would polish them anyway. This will help to keep the seal lubed so it last longer. I haven't had to tear apart a dana 20 case yet. Guess that means they last forever. I replaced my Quadratrac chain once and did all gaskets and seals while I was at it, but the D20 should be similar. You shouldn't have to dismantle the case.
This procedure should be the same for the rear axle seal too.