yea 20R and 22R are the exact same block besides the size of the piston, R151 is the baddest tranny of them all guard it with your life
but it wont bolt right into the truck , your tranny shifter will come out where your tcase shifter is now and you'll have to cut your driveshafts, shorten the rear and make the front longer
now what you could do is talk to cruzerdave or Marlin and trade that R151 in for a lot of $$ off the price of a L52HD it will bolt right into the 79 with no modifications
