Do you want to fly a Hammer .40? If you add length to it then you're not flying a Hammer .40.
A lot of the old designs are short by today's standard, I'm not sure why? Some of the engines back then where a bit on the vibratory side so maybe they had to shorten the heavy fuse a bit to keep the weight down? Maybe they were influenced by full sized planes? Who knows, but they didn't seem to start lengthening planes until the mid 80's.
The plane won't fly "wrong" with extra length and if you build it light you may not have to add much extra to the nose as the CG will most like have to move further back too unless you make the stab area smaller.