I would imagine that it can be done but it won''t be easy or cheap, and I am not sure that you will like the results. Structurally, I am assuming that you have a typical IOR era bolt on lead fin with or without an aluminum sump. (I am figuring lead since iron would have horrific galvanic action problems)
During the IOR era these keels were generally cast with integral keel bolts with a hooked end on them. In other owrds, these bolts would extend almost to the bottom of the lead and would have a hook rather than a nut that held the bolt in place. If you simply cut off the end of your existing keel you will risk cutting through the bolts and so will not have a good connection any longer. So to begin with you are probably talking aboat a new keel casting as well as casting a bulb.
The nice thing about an aluminum boat is that you can cut and weld almost at will, which is expecially important in this case.
You will probably have to rework the internal structure. Most aluminum IOR era boats had a system of fore and aft longitudinal stringers and athrwart ship frames. In the bilge at the area of the keel the athwartship frames (called ''floor timbers'' or simply ''floors'')were generally larger than the frames fore and aft of the keel and mast step. There was also typcially a keel stringer on either side of the bilge that distributed the loads fore and aft. To lengthen the cord of the keel you would need to add additional floors and lengthen the keel stringers. Some IOR era aluminum boats also had a space frame and that may need to get completely reworked as well. Its a lot of work but it is do-able.
The biggest concern is the sailing ability of the finished product. IOR boats generally were tender,comparatively heavy and undercanvassed compared to non rule boats of the same era such as the J-35''s or Farr 11.6''s. They used a hull form that was very dependent on crew weight on the rail. It was less than ideal for general purposes. So here''s the problem, you need to maintain at least the original stability, and ideally get more stability than the boat originally had. If you are going to do that in a shallower keel you are going to add some combination of more weight and more surface area. These were already under canvassed boats so you will be giving up speed to the drag of the greater wetted surface and the greater imersion and the increased weight. So then you need to look at the rig
a bit. and if you look at the rig
you may need to look at the structure supporting the rig
. And so goes a viscious circle.
So like I said, Its not easy or cheap and you probably won''t like the finished product.
BTW How big a boat is this anyway?