The 454 rubber mounts were different on my 79 from a SBC to a BBC
The rubber mounts are the ones that bolt to the frame-----both sets of holes should be in the frame for these.
You will need to change the exhaust to meet up with the BB.
You will need a LARGE radiator (454 trucks got a 4-row), The BB fan shroud, and fan.
Wiring to the front of the motor may need to be lengthend----I was about 2" short for my alternator wiring.
A 454 Flywheel or Flexplate must be used.
Very straight swap with no issues at all-------did mine in an afternoon, and had the exhaust put in the next day (of course I was much younger and had more energy way back then).