They probably all shift nicer than the stock Sunbeam box...and as long as this one doesn't pop out of second and fourth gear...it'll be a major improvement.
I've read, but have no experience, that most of the BMW shifters have a lot of plastic bushings and a lot of slop. A number of after market shifters out there are all steel or aluminum to eliminate slop/flex. I'm honestly not sure I could get a Miata 6-speed in the car without extensive surgery. I certainly can't get the ZF 6-speed in there.
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I shoved this thing up in there today and looked it all over (lots of pics tomorrow). But the long and short of it is...the 260 fits in the trans tunnel, almost, perfectly. But one of the bosses on the driver's side of the gearbox hits. Five minutes with an angle grinder and it won't hit anymore and it should fit perfectly. I'll need a custom length mount to hold the shift lever in place and a custom length shifter linkage, but with that...my shifter will come out in exactly the stock location. I actually plan fab my stock shifter handle to fit. I may even be able to modify the original Sunbeam trans X-Member to fit the Getrag and bolt it all into place; not sure yet, but my eyeball-o-meter says, "It's possible".
And the Getrag's shorter length means I gained some very critical exhaust clearance. I can now run a straight back long-tube header, and use a cross over flex-pipe and slot everything into place. It's going to work out beautifully. At this point, my three biggest hurdles are ground clearance with a K24 installed, properly building and routing a fuel system, and building a rear end for the car.
If I had an extra 5k lying around, I could probably have this thing running and driving before Jan 1. Unfortunately, I don't...but that's okay, I'm not in a huge hurry.