Exhaust leak at manifold

[roundout]

A couple months I bought a KKK turbo off a member here and installed it on my 84 300D with a turbo install kit from Febi. Last night driving 65 down the highway I started hearing a shrill high pitched noise, so I pulled over, thinking that maybe I had a bearing spinning or an accessory seized up. Turns out that the shreak I heard was the sound of the turbo, with sound and exhaust escaping from the bottom of the exhaust manifold where it bolts to the cylinder head. I am pretty peeved that this leak has opened up because the work is messy and tedious and the kit was something like $70. I don't want to do this work over, but it looks like I'm going to have to.

Do we know what might cause this leak to develop? The FSM doesn't give a torque setting for the nuts that hold down the manifold and there isn't room to get a torque wrench in there, anyway. Is there anything that I might have messed up on the install that would cause this leak to open up 6 months later? If it was something I did or didn't do, I don't want to mess it up again.