My .02......
If the bigger tubes cost the car anything in the 60'(footbrake combo), you'll never make it up down track.
There is no way to know how the car will respond to bigger headers than to try them.
Fwiw....... A lot of $$$ was spent on bigger carbs and bigger cam.... Which basically did nothing to improve the ET.  I found the same thing w/ a bigger intake manifold on my 572. The 60' suffered so much that the extra topend power never was a benefit as far as ET goes.
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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