[84LX89GT]

Go to www.cliffordperformance.com they have lots of stuff for the 140/170/200/250 family of engines.
As for your intake manifold, i had an '83 zephyr (with the 200 straight 6) and the intake manifold was welded to the head (it was a bar) and if yours is similar then there's nothing you can do to increase flow except make your own intake manifold. The 250 cylinder head had a larger intake manifold "log" that flowed a little more and could probably be picked up for cheap.

Exhaust work would be a good way to go considering how restrictive the exhaust is and as for a 2 barrel carburetor, clifford sells adapter plates to change the stock 1 barrel to a 2 barrel.
You can pick up a header or split headers for an arm and a leg and have custom exhaust made, but it's not really worth the money. You could also have the stock head extrude honed (www.extrudehone.com) so it can actually flow something that doesn't smother the engine as much.

I know a guy who restored a prototype shelby mustang GT200 (straight six) and the stock intake was cut off and a new one put in it's place.

There's not a whole lot you can do to make power with those engines since they're so gutless it's not really worth your time to try and make one fast because anything you do without major changes to the intake won't yield you a whole lot of power for your dollar.

Good luck,