You will need different motor mounts
In addition many other things will need to be changed -
Fuel line routing
Radiator & hoses
Drive shaft as the 289/302 is a different length
Front springs if original Besides being worn out they will be lighter for the 6
That year all units got 14" wheels which should be 5 lug. They will be drum all the way around and need to be in good shape for the added power. Stock auto trans would work I believe.