Frame modifications

[wldpilot]

I have a 1940's Harley frame I intend to use for my build.  Er, more like half a frame, which is where my question comes in.
Everything aft of the seat post is missing.  Rear legs, etc.  The backbone is also gone.
Intend to build it back, but not to perfect factory spec's.
The backbone would be shaped slightly different to better accommodate my gas tank.  The rear legs more or less stock, but perhaps lengthened slightly.
Would this place me in modified, or special construction?

[Stainless1]

Modified is my guess if you don't get too crazy.  But as long as it is already needing a little work, you could go long and low, special construction is not a bad class unless you plan to put it on the street sometime.

[wldpilot]

I'm not opposed to long and low, or special construction.  Just trying to find where the cut off is.  I would hate to build following special construction rules, and then find out I really fall in modified for instance.

[wldpilot]

Aha.  I see in the newest update to the rules there is an addendum for ten percent or less increase in wheelbase for modified.
This answers the question to "long and low".  If under 10 percent I'm in modified, if over then it's special construction.  Correct?  This would make sense as per the "purpose built" designation in my mind.