Guys,
Some advice/opinions on this: build a 425 or a 455. Mild street driving, yet want to keep the OEM stuff, eg exhaust manifolds (W30), performer intake, blah, blah, etc. Potential use in my '68 442. Thoughts. pros cons. Thanks.

 

They both use the same parts except for pistons, rods, and crank (and maybe cam/lifters, depending upon the year).
The 455 is a stroked 425, so it has more torque with similar parts.

 

Build a bitching E block 400.

 

That was my plan years ago - a 425 is essentially an overbored 400E.

 

Either stock 425 or 455 will be faster than a "G block" long stroke 400.

 

So, I assume either one, the 425 or 455 would be good. Obviously, the 425 VS 455 is leaving 25 cubic inches on the table. However, that said, Im quite confidant there really would be no performance difference (or very little) between the two engines...in my car. Btw, dumb question: what heads were on the 425s? Again, thanks in advance for the help.

 

425's came with A heads in 1965 and B heads in 1966 and C heads in 67.
For the 425 I found that good piston selection is limited to .030 over. (Mahle Pistons 197816355) You can go .060 but hard to find a real nice piston unless it's custom.
With the 425 you'll have to verify if the block is a 39 or 45 degree lifter angle and the size of the lifters. If the block is 45 degrees then your looking at a non-standard cam grind. If the lifters are 0.921" then you'll have a harder time finding a quality lifter in either hydraulic or hydraulic roller.
If you can figure out the piston and lifter situation though, I'd say overall they are about the same to build. Most people just go with the 455 to gain the extra cubes.

 

455. Longer stroke will be a positive as you'll be operating on the street and cubes and torque will be what you want. Actually, for your intended use the original G-block would be just fine if you still have it kicking around.

 

455 much more common to find produced 1968 thru 1976. The 425's are around though many pass them up for the 455...

 

The 425 is a better engine. Change my mind.

 

Okay ... for OP's intended use the 455 is better,

 

4 1/8” bore x 4” stroke, 7” rods, and forged crank. What’s not to love?

 

3" mains.

 

Not possible to change your mind or anyone else's that truly understands Oldsmobile's legacy. There's only one reason that more migrate to 455s outside of originality and availability. And its due to the old adage "There is no replacement for displacement". A concept that only holds up if all else is equal. Which is not the case here.

At the end of the day the pro engine builder can deliver the same horsepower out of both, and the 425 can naturally handle it better.

 

I can't. Because you're right.