OK - here goes. I've got a TKO waiting to go into my XD.
Firstly, the input shaft is longer than the toploader, therefore you can't use your toploader bellhousing by itself. There are 3 options for you to choose from.
1. Buy new custom bellhousing to suit tremec/351C: $350-400
Dellow Automotive in NSW - http://www.dellowauto.com.au/
2. Buy billet aluminium spacer plate from Mal Wood in QLD ~$250 from memory.
Goes between toploader bellhousing & tremec grearbox.
3. Nut case option - DON'T DO IT - machine input shaft.
Speak to the guys from the above shops - they will both tell you their option works well. I bought a new alloy bellhousing from Dellow's (they have both cable & hydraulic clutch options).
Thing's I'll be definitley checking after the bellhousing has been fitted is:
1. Gear box mounting face on bellhousing is PARALLEL with block/crank.
2. Concentricity - bellhousing will mount the gearbox input shaft EXACTLY in line with crankshaft. Both checks can easily be done with a dial gauge - see link.
These important measurements will ensure long gearbox life/less worries (wearing bearings out/destroying spigot bearings in tail of crank etc). Big $$$ boxes to just slap up and install without proper checks.
Your standard toploader 10 spline clutch disc will fit your 3550 (TKO is 26 spline). I'd use a new throwout and spigot bearing. You currently using a single/twin plate clutch?
The output shaft is the same as the C4 & toploader so that should fit. May be slight issues with driveshaft length - should be OK. (TKO is 31 spline).
You have a shifter? There's the standard tremec version or plenty of aftermarket options.
Your speedo sender should work but DO NOT USE TOPLOADER SPEEDO COGS. Tremec ones are different - Can order from Dellows/Mal Wood.
Here's some links that provoide some good info on Tremecs:
Thats about it for now! Do you already have the gearbox? or buying a secondhand one?
Here's a Tremec TR-3550/TKO instruction manual for you!! (pdf file)
Tremec Home - http://www.ttcautomotive.com
Good luck! Don't hesitate with any other Q's.