If you leave the 454 all iron and you compare it to an aluminum intake'd 350,you are looking at about a 250# difference.The 454 in the pickup is not a hot rod engine,but it will still outrun a stock 350,and it will also bolt right into place of the 350 on the same mounts as your 77 monte already has.It will bolt to the same trans too.The bigblock is longer in how it hangs foward of the motor mounts so the fan will be closer to the radiator,but since there is about 3 feet of dead air in front of the fan on your Monte,I would bet that this wont be an issue.There is a bigblock specific fan shroud for bigblock monte carlos since it was offered up until 1976{your car is the same as a 76 only it was never offered with a bigblock},but if you cant find one,I believe you can modify the stock piece.If your fan shroud has ribs running around it like I remember some of them having,you can usually get away with removing just the material beyond the last rib and presto,you have a bigblock fan shroud.
As far as I know,all the 73 thru 77 GM mid size cars had A/C boxes that would clear a stock bigblock with low rise calve covers and exhaust manifolds.The truck manifolds will fit your swap,but you can also use headers from a 70 thru 72 bigblock camaro.If you buy the Hooker compitition headers for the Camaro,it will come with instructions on how to notch the engine mount bracket on the later engine mounts so that they dont hit the headers.You have to do this mod when you use those headers on 73 and later Camaros too.The job can be done with a hacksaw if you really had to,but a sawzall makes it way easier.
With an aftermarket intake and tall air cleaner the bigblock may not fit under the stock monte hood.A good intake for this swap if you have no money would be a early 70's low performance 402 intake since they have no EGR provisions and are very low profile,and they are also a large oval port which will help flow despite the mismatch,and they are usually sold on Ebay for under $15.An Edelbrock Torker will fit under the stock hood too,but it isnt the best bigblock intake on the market,but it is lighter then a stock iron intake and will perform better.Dont go out of your way to buy one,but if you want a low buck upgrade,I am sure one will turn up on Ebay within the next few weeks since these intakes are always floating around for cheap.add a 750 carb to it and with headers you are already making way more power then any smog 350 with similar mods could make.
I hope you have all the brackets from the bigblock in the truck since you will also need the altenator brackets and power steering and A/C brackets to make this all work.If the truck had a hydroboost brake booster with a remote resivior,you can use it if you like by bypassing just the lines that go to the booster,or you could bastardize the stock smallblock pump by bolting it to the bigblock brackets.I think you need to remove the pump pulley for this job which is a bit of a pain in the butt.
If the 454 is a good running engine that is mechanically sound,you would rework it a bit while it is out and end up with a lot more power then you might expect.I would tear it down to a shortblock and replace the oil pump.Have the heads cleaned up and a valve job done.Buy an entire cam kit with springs lifters and timing set and swap it in along with new stock stamped rockers.I would try a cam like the comp 268 degre hydraulic since this is as big as you want to go with the smog heads.With a used torker intake,750 carb,headers,and a good tuneup to the stock HEI ignition and this engine will be a bear.If you do nothing else,replace the timing chain and oil pump with new stock replacement units since these are 2 items that can make a good engine die an early death when you drive it with a lead foot.
Good luck,and let is know how you do with the swap.
