Couple of options on this. First, what type of bellhousing do you have? I have a 427 so FE engine with a Lakewood scattershield and a Tremec TKO 600. The TKO is same dimensions as the 3550.
The stock, long input shaft that comes on these requires a spacer plate. Or, you can buy a shorter input shaft and replace the original with it. Then you don't need a spacer.
However, note that different bellhousings can be different depths, and you need to be accurate or what happened to me will happen to you - you cram the input shaft up into the crank and then chew up the thrust bearings on your crank when you try to turn the engine over
I originally had a cast iron Ford bellhousing and found (the hard way as mentioned above) that it was 1/2" deeper than the Lakewood.
Forte's Parts - (forteparts.com I think) sells the shortened input shafts and adapters and gizmos for just this type of thing. Good place to look for starters, and also post to the Transmission Talk forum here.
Good luck