I am putting a 454 (bored and stroked to 496) in my car but it is not an easy job. Firewall modifications or driveshaft lengthened, custom engine mounts, special (expensive!) headers, special oil pan, etc. etc. etc.

If you want a direct bolt-in upgrade, go to a SBC (Small Block Chevy) like a 350, or if you want to go a bit better, a bored and stroked SBC = 383 cubic inches, will give you easily over 400 horsepower. These are available as 'crate motors' from all kinds of places at reasonable prices with all new internal parts (much better than a rebuild).
http://www.competitionproducts.com/prodinfo.asp?number=LB383CD

All you will need to do to install a SBC is get the correct bellhousing becaues 6 cylinder and V8 had different bellhousings (I bought mine on eBay) and motor mounts - I recommend staying with the original type front mounts
http://www.classicchevy.com/product.asp?pf%5Fid=18%2D08&dept%5Fid=2209
http://www.classicchevy.com/product.asp?pf%5Fid=18%2D06&dept%5Fid=2210

Or you can change engine and transmission mounts and use ANY chevy bellhousing but you will have to add a tranny crossmember, custom exhaust, etc.

Advantage of SBC over a 454 are
(1) your car could have had a SBC as original equipment - BBC not introduced until 1965 whreas SBC introduced 1955
(2) SBC parts are WAY cheaper and more readily available, especially high performance parts
(3) BBC weighs more

There are some details like radiator and fan location but that is the basics.

Let me know if you want more information.

Dave

i Put a sbv8 into my 55 gmc and i went with the mount on the front of the engine. i do have a spare set that have to be bolted to the front crossmember. i used a bellhousing out of a late 60's truck for the rear mounts. pretty easy just have to source the parts.

You can make or buy a set of side motor mounts cheap. If you're not that great of a fabricater I would suggest buying them.  I usually use an empty engine block and an empty trans to get the engine to sit where I want.  Basicly its a matter of getting the engine where you need it, if you are using HEI you need some room at the back of the block. I generally use the 3/4 ahead side mounts.  After the engine is where you want, measure for a trans crossmember and driveshaft.  You will be pretty much on your way ater that. Here's a shot of the engine with side mounts and a shot of the trans crossmember.  I made my own but you can but universal crossmembers at a reasonable price.  Good luck!!!