correct. The Diesel FWD rules have their own rules starting on page 64 of the 2008 NTPA rulebook. which lists the rules that pertain to the diesel class. However, rule number 1 on page 64 states: All normal FWD rules apply with the following additions.
With that being said, the normal FWD rules that pertain to engines are listed on page 62:
letter D number
1. Vehicles are limited to a single automotive-type engine.
Note: Automotive engine is any engine or its replica available in a passenger car. Maximum (8) cylinders. A replica to be considered legal must accept and swing a stock crankshaft.
With that stated, the answer is yes, you can run a billet or aftermarket block in the diesel FWD class as long as it will accept and swing a OEM crankshaft for the engine that it replaces.