I like the 4x18 as an exercise, whether plausible or not. It would result in 2 or 3 "higher power conferences" and 1 or 2 "lower power conferences." I'm going to propose a 3x24 below. My 72 would include the current power 65 + BYU, Central Florida, Cincinnati, Houston, Memphis, South Florida, and Temple.

B1G
Pacific: California, Oregon, Oregon St, Stanford, UCLA, USC, Washington, Washington St
Central: Arizona, Arizona St, Colorado, Iowa St, Kansas, Missouri, Nebraska, Utah
Midwest: Illinois, Indiana, Iowa, Michigan, Minnesota, Ohio St, Purdue, Wisconsin

SEC
West: Baylor, BYU, Houston, Kansas St, Oklahoma, Oklahoma St, Texas, Texas Tech
South: Alabama, Arkansas, Auburn, LSU, Mississippi, Mississippi St, TCU, Texas A&M
East: Central Florida, Florida, Georgia, Kentucky, South Carolina, South Florida, Tennessee, Vanderbilt

ACC
North: Boston College, Michigan St, Northwestern, Notre Dame, Penn St, Pittsburgh, Rutgers, Syracuse
Atlantic: Cincinnati, Louisville, Maryland, Memphis, North Carolina St, Temple, Wake Forest, West Virginia
Coastal: Clemson, Duke, Florida St, Georgia Tech, Miami, North Carolina, Virginia, Virginia Tech