Further manufacture: Tractor to wrecker after Jan. 1, 1998?

We understood that after Jan. 1, 1998, chassis modifications that did not exceed 75% of the cost of purchasing a similar new chassis would not be considered further manufacturing. However, in Technical Advice Memorandum 9852001, dated Aug. 25, 1998, the Internal Revenue Service (IRS), without reference to this 75% threshold, treated the modification of a truck tractor into a wrecker vehicle as further manufacturing. Is the 75% rule still applicable for determining whether a chassis has been further manufactured?