Well the question you need to ask yourself is do you want a Pontiac engine in your Fireturd or an Oldsmobile engine.
Obviously the 400 is already in place, everything fits, and although the 400 is not the best Poncho engine you could start with you could certainly do worse. Also performance parts are readily available for it, if a bit expensive.
Stabbing in a 403 would require figuring out mounts, accessory fitment, etc. However with some junkyard digging you can come up with everything you need. Also if I remember correctly it is a somewhat heavier engine. Add to that generally speaking performance parts are more expensive and harder to find.
So the call is really up to you. If you really want an Olds and have the time, money and skill to make it work, go for it. However if you are lacking in any or all of those areas stick with the 400.
Now for my opinion. Don't build either, find a Pontiac 455. Unlike the 403 it will drop right in the hole and everything will fit.
Thanks for the pic, jedimario.
"Everybody believes in something and everybody, by virtue of the fact that they believe in something, use that something to support their own existence."
Frank Vincent Zappa, 1940-1993