Tom Egli on the subject of foils. Taken from an email. Brand names have been changed to protect the innocent.

For the foils, all three types are relatively good. The Brand X foils are injected (sort of, but not quite, like the Laser foils), which gives them a good a good shape and fair finish since they come from a mould, but tend to be relatively flexible which is good if the crew is light (if flexes and loses power too easily with heavy crews) or is a lot of sailing is done in heavy air (where you want it to flex and lose power in the gusts).

The Brand Y foils are quite nice, but are still made by hand in that quaint and idiosyncratic British way, which makes their shape dependant of the skill and mood of the individual on the shop floor, although my elliptical rudder is very fair and symmetrical.

The Brand Z foils are making a bit of a comeback on the market after a couple of quiet years, but I do not know enough about them to comment.

If you check the width of your centerboard case, you'll probably notice that it varies by about 2-3mm along the length, bu you only have to focus on the contact area of the head of the c-board when it is down (probably a 1-1.5mm variation for an older wood boat, 1mm for a newer glass boat). Order on the shy side, since it is a bitch to widen a centerboard case (trust me, I've tried), or to shave down a glassed board (especially the Winder). Another benefit of going a bit narrow is that you can build up the sides of the board with a epoxy-graphite mixture to reduce friction and increase abrasion resistance (sand and grit always get into the case) or by using my favorite technique and laminating teflon pieces to the board. Go to a shop that sells curling supplies, and ask them for the sliders sheets for shoes. These are sheet aproximately 4"x14" that are chemically treated on one side to be able to glue the teflon, and they come in different thicknesses. The bolt hole location should be determined according to how your boat currently handles.

If you're happy with your rig rake settings, but can't seem to get rid of the weather helm, you have to move your board back to get the sail and board centers of effort closer together (i.e. move the bolt forwards). The fashion these days is to sail with much more rake than in the past to improve upwind performance, so I suggest placing the bolt hole as far forwards as possible anyways. If that ends up giving you lee helm, you can step the mast a little further back (which is a good thing in older boats since they lack a little bit of buoyancy in the front). All of the above is assuming that the new centerboard generates a lift vector at approximatly the same place as the old one (usually the case for the type of sections used for centerboards).