Summary of Olds 425/455 Engines

You wish:
0- Exotic heads: D, F, H, or maybe K. Block? Crank? If the price is right, who cares!?
Best of the best:

Toro/442 engine 1968-69. Big valve C heads. 4V carb. High compression. Toro engine has oil pan with extra capacity, built-in crank scraper, factory windage trays. The best heads, crank, block, etc. Best-flow C heads, will need hard seats installed for the exhaust valves. Small, inexpensive lifters on the most common bank angle.
Toro/442 engine 1970-71. Big valve E or G heads. 4V carb. Same benefits as above, plus hard seats. Generally equivalent to or better than the comparable 442 engine. 1971 G-head engine was much lower compression than previous years due to deeply dished pistons. Points off: C heads are said to flow best.

Best, easier to find:

Toro/442 engine, 1967. Big valve C heads, all the usual Toro goodies. 4V carb. Will need hard exhaust seats. Some racers favor the Toro D block's large lifters, with the modern 39 degree cam bank angle, because the large lifters seem to tolerate high cam lift better. 400 CID E-block 442 engines are very rare, easily tradeable for a nice 455.
Slight drawback: 425 internals [or, is that forged crank and short stroke a bonus?], requires the early pattern flexplate, and those 0.921" lifters at $100 a set or so, which require shorter pushrods. Also, since no Toronados were MT cars, if you are using a manual trans, you will need to have the crank drilled for a pilot bearing. But, then, that applies to almost all Olds engines as well. If you'd rather have big valve heads and a forged crank than 30-55 more cubes, this is the engine for you. If you'd prefer cubic inches, but can't find the best, or the next best . . .

Any [i.e., non-Toro] 1968 to 1969 455/400 is probably the easiest to find. Common in 88, 98, wagons. Will have C heads, probably not big valves. Engines in Vista Cruiser wagons may be the rare G-block 400 [serviceable as a 442 engine] and may have 'notched' rocker arm covers and/or dual-exhaust manifolds useful on a Cutlass/442. Of course, a 4-barrel engine is preferred over a 2-barrel, but if you are buying an aftermarket intake and new pistons, that point is moot, except that you can sell a 4V intake and carb more easily.
1967 425 engine. Common in 88, 98, wagons? "Best flowing" C heads, though not as good as big valve C's. Forged crank. Drawbacks: Non-Toronado 425 D-blocks with 0.842" lifters will have the older 45-degree cam bank angle; cam selection will depend on this feature.
The usual 425 vs 455 tradeoff: you will need to either
A) build a 425 forged-crank engine with these parts, or
B) find a 455 or 1968 to 1969 G-400 engine to donate the crank and rods, then get 455 pistons. 425 pistons are not fungible with 455 pistons due to a different distance from pin center to top of piston ("compression height").

Good:

1966 Toro/442 engine. B heads. Identical in benefits and drawbacks to the 1967 Toro engine [#3 above] except that the 1967's C heads are said to flow best.
Any other pre-72 [head up thru G, but not Ga castings] 400, 425 or 455. Run of the mill engines such as a 2V carb would likely have low compression, small valves, no windage trays, etc., all of which can be altered during a rebuild, for some cost. If it's a D-block 425, watch out for the small lifter/early cam bank angle combination.

Darn near adequate:

1973-up smogger 455's with J heads. If you buy high compression pistons, have larger valves fitted, buy a nodular crankshaft, an aftermarket or pre-EGR intake, and a windage tray, you'll have a motor that's almost respectable. But, it'd still have J heads with that tiny exhaust port. Why bother? Avoid, unless there are no alternatives.

[ Thanks to Chris Witt for this information ]
Please refer to the Blocks section as well!
Please refer to the Heads section as well!
Please refer to the Cranks section as well!