The bitumen doesn't need to be removed as there are adhesives, as Pete links to, that will cope with it. It is better if you do though.
The reason is when surface sanding when laid, there are less "steps" in the floor and therefore less sanding to be done, this will give a better finish.
It's less of a problem if the area is small, but can still be a pain.
With reclaimed flooring, the cleaner the blocks are the better and easier they will be to fit.
How big an area have you to cover?
Sorting through the blocks to start with is the way to decide if you need to even up/plane the blocks. Get as many as you can all the same size or as near as possible, then keep the odd sizes for cuts. We're not talking in 10ths close in size, give or take a mill and that should do.
You will get good results and an easier job if all the blocks are from the same floor, if not there may be some work to do.
As always with wood get it into the room where it's going to be used for a few days (...sucking eggs and all that), and make adjustments to the size of the blocks if needed after a few days or when fitting. A helper will make the job soooooooooo much easier.
Adhesive from the Sika range was always my first choice, and use a notched trowel for spreading out the adhesive.
If you can get hold of one a roller (heavy sort) to set the flooring down into the adhesive, you'll be surprised how effective a roller is.
What pattern are you laying the blocks in?
