I've had my 83 hurst olds back on the road for about 3 months and I'm getting tired of the stock 307. Trans is a 200 4-r with a shift kit and 5k miles, rear is an eaton 3:73 (HD)
Here are the question(s): is trying to locate a 350 olds worth it? if I find a 403 how likely is it I will need machine work? Is an LS swap total blasphemy? Moreover, would it be a nightmare parts wise?

This car is my daily driver and 275-300 hp would be more than enough for me. 3k is my budget and I'm looking to gather advice and not a lecture. The stock motor has 92k and runs like a Swiss watch..consider me greedy for power

 

If by 'worth it' you mean will it make a noticable difference over your stock 307, then hell yes. Even a bone stock late model smogger Olds 350's ( the ones with the 3A heads ) would have more power and if you do a few mods then skies the limit. An early model 350 in good running condition with nothing more than a cam swap, some bolt-ons, and a good tune would run circles around a stock VIN 9 engine.

If you're planning on doing a full on rebuild the correct way, every motor's going to need at least some machine work.

A Chevy motor would be blasphemous. An LS ( IMO ) isn't really a Chevy engine. It's more of a generic GM engine. Every time I look into an LS swap into a G-Body just to get a feel, I get frustrated on exactly what is needed to do it right without hacking/butchering things up. So basically, it's far from a simple drop in swap and it can get pricey too. Since your power goals are modest, I wouldn't even consider doing an LS swap.

You will be hard pressed to do a full on rebuild for that unless you're doing the work yourself and/or can get a good deal on machine work and parts. 3k can go a pretty long way if you're able to find a decent engine that doesn't need a rebuild though. This might be a good canidate for you; http://lasalle.craigslist.org/pts/3845378384.html