the 390 will make gobs of torgue,but the change requires new frame to motor mount brackets,also keep in mind that the rear motor mount[trans mount]stays constant,all of the drive line,shafts,clutch,and linkages stay lined up,no matter what you put in,i agree to rebuild the m series,use later heads with hardened seats,some trick stainless valves,just watch the lift of the cam,and coil bind,the [FE] passenger,light truck had no hardened seats,if you do put a 390 in,have the exhaust cut for 428 cj specs,will not lose bottom end,and use STAINLESS valves,then you have a set of very close 406 heads,202 ins,from 156,to 165 on the exh,also USE HEADERS!,MAKE sure THE EXH PORTS ARE RIGHT,there is a 1/2 in dif in the ports,not the bolt pattern,i also found the crane power max 260 cam[fe]whips up some bottom,andVERY STRONG MID RANGE,I BEEN RUNNING IT FOR YEARS,CAN LITE UP 36" TIRESalso break axles on shifts,also on the [fe] check head to intake match,large port heads can use small,or large hole manifolds,small port heads cannot use large port intakes