But spending other people's money is one of those things I do really well
One of my favorite Bulletin Boards for Ponchos: http://forums.performanceyears.com/6...m&s=7286011111
And I like this place for engine hardware just because Ken is good about talking with you about what you want to do with your motor. Of course the same can be said for most of the widely known poncho shops- it's a rather welcoming community they seem to like newcomers as it's one more poncho on the road.
As for the new GM big cube motors- while I haven't personally tried to drop one into a poncho's engine compartment, I highly doubt it'd line up. It was a really big deal when a non-GM affiliiated company started turning out aftermarket blocks patterned after the Pontiacs- admitedly it was an aluminum block but it was still the first new "Pontiac" blocks since 1980- I think.
Don't underestimate your 400 it can be a very solid motor and would theoretically be cheaper since you already have it, since finding a 455 would require putting down $$ for the block and crank, if nothing else. The machine work would be mostly the same so if you fork over cash for a new block it certainly is that much you can't spend on something else. A Poncho 455 is simply more cubes and would drop right in. I can't honestly say when they stopped making the 455 but I don't think they stopped in '74 I think
they made 455s up to/through '79.
If you have the cash to stroke your 400 it can be done for a "reasonable" cost- you've already noticed that reasonable in Pontiac terms isn't quite the same as small block chevy terms. There are also new aluminum heads being produced for the Poncho- and not just Edelbrock there are a couple companies in it now, that you might look into.
Keep in mind also that the stock iron heads for the 350s 400s and 455s readily interchange. You could get some huge chambered heads that would easily let you run 87 octane in a 400 stroked to 455 and still get yourself a decent compression ratio.