I run Doug's 2.25" cutouts on my 68 GTS and they help quite a bit under any conditions especially drag racing. It has a 511" low deck with a n20 grind solid roller.
I'd be cautious taking advice from anyone without direct experience on the matter.
If I was starting all over I'd build the exhaust 3" mid pipes to 2.5" tail pipes with an H. I'd run 3" electric cutouts too.
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