Pretty easy swap, i didn't do it myself, but i helped my friend who built it, swap out a 383 for the 454, so i learned a lot about what is involved, got lucky when he transferred to Georgia shortly after, and i bought the car from him.
Quick checklist, i have a 1978 mark IV 454 in a 1986 monte ss
Induction, since there is already a 350 in there you probably aren't using the computer nor the computer controlled carb, so carb should be bolt on, depending on the intake you use, your hood might not fit and you will need one with a cowl, factory fuel line is fine, and there is a return if you want to use it. I have a holly mechanical pump, edelbrock torker 2.0 manifold, holley 750, a 4 inch k&n air cleaner, this setup needed the 3 inch cowl hood with a 2 inch air cleaner, fuel pump gonna depend on your motor, mechanical be bolt on, if ya need to go electric not sure if the factory fuel line is up to snuff, probly fine for the 7 psi a carb needs, or maybe one of those new remote sump electric pumps would be best, the turbo buicks were electric pumps so using their fuel system is another option
Ignition, hei distributor fit kinda tight to the firewall but fits, swapped it for a mallory mbi when my hei started acting up
Cooling, factory radiator will be marginal at best, got the car with an aluminum be-cool radiator and dual flexalite? electric fans, always ran warm to hot when stuck in traffic, electric fans crapped out in a year and i put an 18" mechanical flex fan on with the factory fan shroud and it never runs hot now, creeps up a bit to warm at extended idle, added a pusher fan to try and help and for the a/c, but not sure if it does much, but pretty sure my warm idle is because my headers are not coated or wrapped and get really hot
engine accessories, used the 78 power steering pump, buddy took it apart to swap in the 86 metric pressure fitting, a/c compressor, swapped in a sanden compressor, converted to r134, i have a one-wire alternator 100 amp, edelbrock water pump and 78 starter
exhaust, not sure if any factory manifolds would fit,the super comp headers mentioned earlier are made for this engine swap and work very well, had to bent the brake lines a bit where they leave the master cylinder to clear the tubes, comes kinda close to p/s pressure hose, some plug wires get burned had to boot a few, next set of headers i install will be coated
rear end, factory 7.5 designed for a 180hp 305 is not very strong, that being said, i beat the snot out of mine all the time with only a stud girdle for added strength, i do get that "grenade with a pin pulled" feeling
tires, i have stock size up front, 275/60/15 on back with 1/4 inch spacers, recommend drag radials, you'll wish ya had them when your rear tires want to change lanes on the 1-2 shift, and it really helps launch control, just dont go over 35 in the rain, nittos seem a little better in the wet/last longer but the m/t et/streets grip better
wiring, not sure what my bro did, he had already ditched the computer/cccarb, had the 1-wire alternator, and the hei distributor wired for the 383
front springs were mentioned, stock springs not rated for the weight, my friend called one of the suspension suppliers and he gave them his specs and they sent him the springs, dont remember the details
stock brakes are marginal, mine are all stock, yeah, marginal
that's about all i can think of, my motor is a moderate build, .30 over (462) 10.5 compression, "biggest cam that would allow power brakes", ported and polished 427 heads, roller tip rockers, not too wild, wild enough for a cast crank mark IV, but wow, SUCH A BLAST TO DRIVE, my friend figured a car released in 78 (G-bodies) was maybe designed earlier for the possibility of a big block and that's why they fit in there pretty easily, not a hard swap at all, and well worth it, car came with a 12 second time slip on street tires, 12.20 i believe, with the drag radials it is a different car, goes like a slingshot, my only friend i couldn't beat had a WORKED mustang with a 200 shot, and the only stranger was this little econo-box which sounded like a jet taking off when his turbo spooled-up (probably has more money in just his turbo setup than my whole car), i cant remember what a small block monte feels like, but the big block just feels right in there, like it should have come that way from the factory, you will never regret it (with gas at this price), with the cowl hood and no monte badging people always think it's a grand national, especially when the flex fan was new and had that flex fan whine, and of course after i just blew their doors off....
sorry for the length of post, car has been down for over a year, mostly due to laziness, this is what i needed for motivation, man my right foot is twitching just thinking about it