Sorry--I should have specified that this is strictly a street car.  No engine rebuild in the plans.  Also--I used the intake/heads example to gauge the futility of trying to make more grunt using the stock logs.  From the responses thus far, I'm getting the distinct impression that without headers, I shouldn't bother with any engine mods other than a re-curved distributor.  And I'm perfectly fine with that--guess I was hoping for an easy way out
 

Greetings all--new to the forum and to the FE.  Subject line pretty much sums up my question.  Given all of the horror stories I've heard and read regarding header fitment on a Thunderbird, I'd really prefer to stick with the stock exhaust manifolds and have a proper 2 1/2" exhaust system with h-pipe built for the car.  BUT-- since so many folks here and elsewhere believe the stock logs are such a choke point--I wonder if they would largely negate the impact of installing an upgraded intake like the RPM and a decent set of heads, even with a free flowing exhaust from the manifolds back?  I'd appreciate any and all insight!