Winston, I put a 4-spd in our 85 300D, and had the same problem, front end came up. This causes a problem driving on the highway, the front end wanders. When it comes up from the weight difference, I think it throws out the geometry of the steering.
Right after I did the swap we drove to Portland, Or. a good 750 mile drive. was an interesting drive, kind of like in a cross wind sort of. Went to see family, but the main was the GTG out at Mc Minville and the Spruce Goose.
Look at the third picture in the following link, you will see how high ours sits compared to the others on each side.
http://www.peachparts.com/shopforum/2227118-post103.html
When we got back home I sourced a set of 240D springs, that dropped her down and she sits lower and ride good, but a little soft when on a mt. road.
I read to throw in a front Sway Bar (Torsion Bar) from a 300TD Wagon. they are a couple MM thicker and will stiffin up the front end. I pulled one a PNP, stripped it down and gave it a shot of POR. Bought new TD Bushings for the mounts and finally got it in. have the engine out so makes it real easy. Don`t have it together yet, so can`t give you a final report.
Here is a thread showing the difference in sizes.
Big front sway bar
Charlie