I just wanted to offer my support for such a project. You are the only other person I've even heard of that wants a land yacht like an sd, but wants a manual too. I thought I was alone. I'm also curious how young you are. I'm currently removing my transmission for repair, which I thought was quite ambitious at 17. Also, my driveshaft could move back and forth a few inches, so it's length is variable. It seems like it should be able to accomodate another transmission quite nicely, but I couldn't swear to it, I've never been under a car with an automatic transmission. Also, are you sure the bell housing would bolt into the engine block the same way an automatic transmission would?

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition