You are right on with the price to do the 400sbc I have figured 6500 to 7000. I'm down to a bare frame right now and will be replacing most of the parts with new. It has been sitting for 20 years (I know). So it does not sound like it would be to much more money for the LSx conversion parts if any. Will be getting new mounts, fuel tank, fuel lines, etc. no mater if it is for the 400 or the LSx. What is the best to look for LS1, LS2, LQ9, or ??
If you're looking at $6,500 to $7,000 for the 400, it doesn't sound like you're horribly constrained by your budget. In that case, and if this is just a weekend driver/fun car - no power adder, I'd look for an all aluminum LSx engine. At this point, I'd also insist on at least a 4" bore. So you're looking at an LS2, LS3 or L92 (there may be other aluminum block truck engines, I'm not sure). Look for a low mileage complete take out including, at a minimum, engine, wiring harness, ECU, all front engine accessories and maybe also the tranny. LS2's and L92's will be cheaper than LS3's.
Pick headers and engine mounts as a unit - headers are designed to work with a particular placement of the engine in the car, and various adapter plates move the engine various distances fore and aft in the engine bay. Get it wrong and there's a good chance you'll have header-steering box interference issues.
You'll also need to pay particular attention to the fuel tank. Fuel injected cars don't have a fuel bowl like a carbureted cars do, so if the fuel pickup is uncovered for even an instant, say going around a corner with less than a quarter tank of gas, you'll feel the engine stagger as it's denied fuel. There is a lot of work being done on the fueling system for LSx engines in first generation F-bodies by the guys who autocross and/or track their cars, so I'd wait a bit before jumping at a fuel tank.
If you haven't already done so, I'd step back a moment and plan the overall build - suspension, brakes, transmission, etc. LSx's will (if reasonably cammed, although "reasonable" is much broader than with an SBC) cruise happily at 1,500 to 1,600 rpm. Put in an approriately selected set of rear gears and an overdrive transmission, and you won't find yourself wincing quite as badly at the gas station. Put in a T56 and you'll also have terrific acceleration off the line.
Finally, you should be able to do an LSx for about the same (or less) than the 400 SBC build would run.