I'm assuming with the 6k budget you're just talking about the engine alone. Not all the misc parts you need for said engine to run correctly ie mas meter,injectors,tuning,exhaust,ign,power adders etc.
What shape is your current engine in? If it runs well ie doesn't burn oil,has good oil pressure,etc. I'd throw a good h/c/i on it with all the above accessories you'll need and make a great running street engine. A well set up 302 with a good trans,suspension,rearend,and tires is capable of easy 12 sec et's in the 1/4 with good street manners. Or throw a good turbo/blower with supporting mods on it.
If your engine is shot and you do need a new long block there are a lot of good shops you can order one from like DSS,CHP,Ford Strokers to name a few. But with a 6k budget you will be limited to a stock block rebuild. With a stock block you will be limited on how much power you can ultimately run. The unwritten rule is around 500rwhp is block splitting territory. If you are planning on some crazy bad*** twin turbo setup you are talking about a 20k build not 6k. You have to remember all the extras add up quickly. A meter,injectors,tuning, and fuel system add up to an easy 1+k depending on what you need(good fuel system alone is 1.5k)