Welding is better than pop rivets.  Sealing the seams is a must, unless you
want to do the job again.  Pop rivets?  Well, if you overlap the seam by a
healthy amount and seal it well it should be acceptable.  Quality of
aftermarket body parts varies, especially when it is not visible from the
outside, like a floor pan.  Don't assume anything about the fit until you
actually see it.

Earle

First, take the time and spend the money to take a welding and auto
body repair class at your local votech.  It will be money well spent.
Stop by the local book store and browse the autobody repair books.

If your just going to offroad the thing and don't care about replacing
body parts every so often then plug on ahead.  But if you are planning
on keeping it for a long time it's best to learn proper repair methods.

You might also luck out and find a local repair shop that is willing
to work with you and help you learn.  That's how I fixed up my old C-10.

Remember that rivets are only good to hold things your going to weld. :)
Rivets usually produce nice rust holes after a while.  Personally I don't
like them.