I am currently looking into a Pontiac Trans am 400 CI engine with the 4 on the floor! however soon i'll have the chance to purchase a 403 for the TA how much more performance would i get, which would be better and if i just bought it and then built it up big and bad and later put it in the TA would it make a difference from building the 400?
I don't think it would make that big of a difference. If you wanted the extra three cubic inches, you could just bore the engine you have and that'll save you some money.
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.