reducing keel/adding fin?
Your question is a little vague so I am not sure precisely whether 1) you have a keel that is long fore and aft and you want to improve performance by going to a fin keel or 2) you have a keel that you would like to make shallower and by ''fin'' you mean wings.
In the first case you are somewhat limited in what you can do. Depending on the specifics of the boat''s design, a fin keel requires very different structural support than would be found on a long keeled boat. Obviously adapting the boat to a new fin would be complex, and expensive, and hard to do so that the resultant boat actually sailed well.
The second case is a little simpler but it too has its ''issues''. To begin with, simply shortening a keel can really have a major negative impact on sailing performance. When a shoal draft version of any particular boat is designed, the foil portion of keel of the shoal draft version is typically substantially larger in area because a disproportionately large proportion of the resistance to leeway comes from the leading edge of the keel. Normally this means increasing the fore and aft length of the keel significantly in proportion to the amount of keel span that is reduced. As a result the whole internal structure of the boat needs to get altered to support this longer length keel.
The loss of leading edge length can be partially offset with an end plate that prevents water from being able to slip off of the bottom of the keel. Designing a properly shaped set of keel wings is an extremely difficult process without the nearly unlimited budget of an America''s Cup campaign. As a result, on most so called wing keel cruising boats, in terms of leeway reduction the ''wings'' act as little more than a high drag endplate. An end plate is generally thought to allow the leading edge span to be reduced by 10% to 15% of its length without resulting in a significant increase in leeway but that assumes equal keel area. Remember that the span of your keel on a 5 foot draft boat is probably no more than 3 to 3 1/2 feet so you are talking a span of no more than maybe 6" and when you add back the thickness of the bulb you are probably back to where you started.
An endplate does little to offset the lost stability from the portion of the keel that is cut off so a bulb is generally needed to offset that loss. Bulbs vary in shape but they need to weigh significantly more than the weight of the keel that has been cut off. As a result they add a lot of drag due to the increased frontal area and wetted surface of the bulb. Properly shaped, a bulb will serve as an endplate. A Scheel keel type of bulb is especially shaped to maximize the end plate effect and minimize drag.
Which brings us to the practicality of cutting down a keel. Traditionally keel bolts were installed in long holes that went almost full length through the keel and had a big nut and washer at each end. Cutting off a traditional keel meant removing the keel and keel bolts, cutting off the keel bottom, shortening the keel bolts and threading the cut of end of the bolt and making new bolts pockets. All in all, it is not all that bad. BUT keel bolts in modern keels are generally ''J'' shaped and are cast into the lead. They often come quite close to the bottom of the keel so that in cutting off as much of the bottom of the keel as you are proposing you run the risk of cutting through quite a few keel bolts. I understand that there is a way to tell where the keel bolts are in the keel, but I can''t really what that method is.
Lastly there is the issue of motion comfort. Even if you can live with the increased leeway, and possible increased weather helm, there will be a pretty noticeable affect on rolling. The keel depth, and area goes a long towards dampening roll. When you reduce the area and depth of the keel, you will end up with a boat that rolls faster, and through a wider roll angle, and which also continues to roll longer than your original keel. This would be a significant discomfort in the rolly anchorages that you are likely to encounter in your chosen sailing venue.
If your boat was adaptable to adding a new keel or a bulb, the hot ticket is Mars Metals bith for expertise and price. They offer a number of stock bulb kits which come in tow halves and that can be bolted on through the bottom of your keel. They also make new keels. Bulbs are pretty cheap, but new keels, especially if you are modifying the internal structure of yoour boat can easily cost 25% to 50% of the value of your boat.
So in conclusion my suggestion is DON''T DO IT! If you were dealing with a deep draft fin keel where the fixes are simpler, the answer might be different, but you are not.