Multiple ways you could do it... Pretty sure every single one of them has been covered here.
Easiest ways to go are Olds small blocks, or Chevy small blocks... As they're optional original equipment in G bodies. All you need is basically the stock parts that went with any G body that had a SBC or SBO in them and you're golden. Should be bolt in turn key swap. If you get the parts to put an olds V8 in, you can pretty much put ANY olds V8 in from the 260 to the 455 without too much of a hassle. Chevy will be cheaper probably, and has more performance parts available... But that's all up to you what you want.
Buick, Pontiac, and Caddy V8's I'm pretty sure all have been done too but they're a little bit more involved. And don't offer that much advantage... Dollar for dollar nothing really beats a Chevy. But if you want to be different, they're all good choices.