It may still qualify as vintage if you can make a strong argument that no significant (and positive = conferring an advantage) changes were made to the engine between pre-1956 (last automatically legal date) and your actual date of manufacture. This is going to look like a legal brief, with exhibits, affidavits etc.
I'd be looking at part numbers, engineering comments, casting dates, and road tests made during the period. Statements from the authors are preferred over references to their work, etc. A 1959 factory parts manual which contains notations such as "Cylinder head, left..... 1952-*" may prove useful.
Why try to make vintage?
Because the engines in OHV pushrod are far more evolved, and have more recent race development.