If you look at
TTI's website it lists the B and E as fitting the F, M, and J's because of the different shapes in the left side of the A bodies. As a matter of fact, our cars have a slight advantage over the B's and E's as we don't have the torsion bars running the length of the headers to deal with.
As far as installing a 4bbl and headers and gaining performance...
You will, but with the stock cam and stock heads the performance gains will be minimum for the money spent. It'll wake it up some, especially with the headers, but remember, the very basic principal of an internal combustion engine is as a giant air pump. More good air in, more bad air out, the more efficient the engine will run (ie horsepower and torque).
That being said, you'll have more air coming in that will hit a bottle neck in the stock heads and cam.
Spend a few bucks and replace the cam depending on your application. Something that will work from 0-5500 rpm and work with a stock converter and springs if just want to give the car a little more oomph. Hit the salvage yards for a late model M body and get the 302 heads off of it and do a gasket match with the intake and headers. Sounds like a lot of work, but then you'll get some more air through to dump out the headers and duals and wake up that little teen to something you'll like for the money.
Edit: sorry, didn't see you're in the UK, so hitting the salvage yards and finding a set of 302's might be difficult for you... but the suggestions still stand: see what you can do to find a set of heads for it or a reputable machine shop that can work with your heads in opening them up.