The European B5.5 are (were?) available with the 100 hp PD engine that is otherwise found in the A4 series cars. With manual tranny too. I would think that a 100 hp TDI in a B5.5 with manual tranny would work very well. We rented one in England last year: smooth, quiet, pleasant to drive, enough oomph to make 90+ mph cruising on the motorway easy. The car ate up the miles. To be true it wasn't as quick as our 134 hp Passat, but it was more than adequate. Earlier Passats (B5) also came with the 90 hp TDI.
The 100 hp engine with a non-DSG automatic would be a real dog though.
My point, after all the rambling, is that the hardware to mount a TDI in a B5.5 or B5 should be available