Let's clear up a few things.
All 260, 307, 330, 350, and 403 Olds engines, AKA Small Block Olds, have the same external dimensions. All intakes, heads, tin pieces, brackets, and accessories are interchangable with a few functional differences. All the bolt holes are in the same locations. Cranks on every small block Olds have the same dimensions as well.
All 400, 425, and 455 Olds engines, AKA Big Block Olds, have the same external dimensions. All the bolt holes are in the same locations as well. There are 2 different stroke size on big block cranks for the record.
The difference between the big and small blocks is that the big blocks have about an extra 2" on each bank, thus the deck(and the entire engine) is about 2" taller overall.
F bodied cars that came from the factory with olds v8's all had a funky looking steel bracket about 1/4" thick in place of the mounts that originally bolted to the engine and sat on the frame pads. I've also seen the funky steel brackets on a few Olds diesel powered cars and post 1975 Venturas/Omegas with olds V8's. To the best of my knowledge, Olds powered F bodies have the motor mounts(frame pads) in the same location as pontiac powered F bodies. I read that(the frame pad location) on another site so it would probably be wise to verify this before going through with any swap of this nature, but I'm positive on the funky steel bracket thing and the small block/big block info. The real key to a pontiac to olds swap is to get a set of the 1/4" steel block mounts. I'd also assume the real important part of an olds to pontiac swap is get a proper set of Pontiac mounts for your Pontiac block. On an olds-to-olds or Pontiac-to Pontiac swap, I'd say use everything from the original setup you possibly could or locate identical factory pieces(or weld new mounts to the frame
)
If you want to build the 403 or any post 1977 350 Olds, you'll run into a problem: windowed main webs. There are holes in the cast iron that leads up to where the crank mounts that make for a weak block. Still, the 403 has a really nice bore to stroke ratio and can be made into a nice street engine. To build it right, use some pre 1972 Olds 350 heads with casting mark 4, 5, 6, 7, and 7
A since they flow better and have smaller chambers than the later heads. There are a few other mods you have to make to the earlier heads but none of them will run a whole lot more than $75 from a shop. If you use an Edelbrock Performer RPM intake for your 403, it won't clear the shaker hood scoop in an F body. The Performer will.
Big block Olds engines have the same dimensions as a small block as far as the bottom halfs go. You can bolt a big block oil pan to a small block engine and vice versa. You can also swap block mounted motor mounts. So, the thing to do in order to get a 455 Olds into an F body is---you guessed it---find a set of those funky 1/4" thick block mounts or just pull the ones off your 350/403 if that's what your F body originally came with. Since the big blocks sit higher than the small blocks, there are issues with the shaker hood scoop. I've heard of people modifying the base somehow to get it to fit, but another alternative is to find an original cast iron one from an early Toronado, although that'll hurt the flow of the intake. THere is a performer available for the 455 olds but no performer RPM.
There are tons of ways to build up Olds engines in T/A's I haven't covered. Look at a site called
www.realoldspower.com or
http://442.com/oldsfaq/oldsfaq.htm for more info.