The block will have to be cut for rod clearances if using the 4.250 stroke. It will need to be cut on the pan rails and at the bottom of cylinders.
Using the 4 3/8 stroke is possible but will require even more cutting of the block pan rails bottom of cylinders and in some cases the inside sides of the block. Unless your shop is experienced with block cutting/grinding on that block for stroker clearances, I would suggest to not try. Most people can do ok even if first time cutting for a 4 1/4 stroke. 4 3/8 gets trickey.
Ed