I didn't realise that the mounting holes were different... Now that I think about it though, I think most of the mid-blocks come with a new engine mount.
Sorry about that.
I put an OS 18 in my nmt, but I haven't run it much since. Only about 10 tanks or so to start the break-in process.
I want to wait until I have the $ for a new header, pipe and slipper clutch before I really start to abuse it.
I think a 21 is overkill though. The diffs are only 2 spider diffs and the dog-bones are so small... I think you might just end up tearing up the driveline. But if you do, I want to see movies!