Steve, unless local rules allow it, it's not NAMBA legal. Here's the rules right out of the NAMBA rulebook:
1. CLASSIC THUNDERBOAT
a. Hull Specifications
i) The boat may be of wood or fiberglass construction.
ii) The hull length will be between 48” – 56”.
iii) The hull width will be a minimum of 24”.
iv) The transom will be a minimum of 10” in width.
v) The hull design will only be one of the following types: round nose, step deck, or chisel nose.
It is probably legal in IMPBA, as shown in the rules posted below:
G. Classic Thunderboat
1. Hull Specifications
a. Boats may be wood or fiberglass/composite type construction.
b. Length shall be a maximum of 56 inches, minimum of 47 inches. Width shall be a minimum of 22 inches.
c. All boats shall have a front or rear cockpit with driver figures from the 1950s - 1970s era, no cartoon or animal characters.
d. All boats shall have a period correct paint scheme and sponsor name & logo. The Sponsor and/or logo may be original or of your own choice.
e. Normal hull configuration shall be conventional round nose, shovel nose, dropped sponson or pickle fork design. The pickle fork shall not exceed 10% of the hull length and be modeled after a classic era boat (1950s-1970s era).
f. No rear sponson, ride shoe or other riding surface allowed aft of the sponson transom. Engine belly pans are allowed. Engine belly pans may not extend beyond the engine compartment.
g. Nothing may extend more than 5 1/4 inches beyond the transom.
h. The strut mounting is optional. Maximum strut length of 3 1/2 inches in length and 1/2 inch in width.
i. A minimum 50% of the gasoline engine must be covered with either a cowling or period correct fake engine. This does not include header and/or tuned pipe.