By 79, there was a difference in the small block & big block mounts.
I know they bolt different to the frame. Clamshells will work just fine.
This is not to say that the small block mounts wont work----everything mounts the same.
You will need
454 flywheel or flexplate
Exhaust will need some work
The 4-row radiator will be a damn good idea, and this will require a HD cooling system optioned upper radiator retainer (Tall radiator---up to the top of the core support----standard sits about 3" below the top of the support).
Upper and lower hoses, all of the front of engine accy mounts.
If you have an electrical oil gage (I doubt it for 77) you will need to extend
the wire to reach the 454 sender.
Might have to do the same for the temp wire (I did not)
Alternator wires will need to be extended a few inches.
I did this in the early 90s----best thing I ever did.
Bought that truck new with a 305, then went 350, 4spd, then moved up to the 454. I still have that truck, but actually quit driving it about 5 yrs ago