Just use your 351 flywheel.
It's the 351/400M motors that were the big block bolt pattern.
I got a 351C from a fellow 25 yrs ago due to him buying it for his LTD that had a hurt 351M in it. Found out it wouldn't bolt up so I bought the motor.
2006/2009 UMTR Points Champion
1964 Galaxie 2dr 390/6-71/4spd
1964 Country Sedan Wagon 428/5spd
1969 F100 428CJ/4spd
1989 F100 302/AOD
1959 B Model Mack