The trans you are considering is a Getrag; there is a slope back version of the iron case trans that is usually referred to as a "slope back".

Look at the front face of both transmissions side-by-side; does the Getrag have the peripheral centering ring of the original? Or just the two dowel pins. The original intermediate plate that is still on the engine may not have provision for the Getrag dowels, and if the Getrag does not have a centering ring there is no way to insure alignment of the input shaft with the crankshaft and clutch.