Right Wil - that is the best way to go
I think Bruce Andserson's book is good but a little misleading in terms of results if I remember correctly...
I would like to see this for a totally stock unmolested 3.2 84-89
Stock dyno run get a baseline.
Add DME get dyno result.
Back to stock then do airbox mod (I think I remember he said even without a filter this yielded little or nothing at all) get dyno result
Back to stock do muffler mod, get dyno result.
Back to stcok do cat-by-pass mod, get dyno result....
Then try them together sequentially and finally all at once.
He just kept dynoing things with other mods if I remember right which was confusing.
All that said I think without going to headers you leave roughly 5 hp on the table with the above mods in place...alone I have no idea as it was not done that way.
Clear?
-Jeff