Nailhead header capacity

I have home made headers on my 401 that have individual header tubes 1&1/2 in leading into a 3 in. collector.
I am replacing this with a 425 bored .040 and I'm told that 1 will need to use 1&5/8 in. tubes.
This is a street machine and probably will seldom reach 5,000 RPM.
Can anyone out there give me some advice here.
I would like to use my exhisting headers on the 425.
Thanks an

That 1/8th inch will make no difference. I guarantee, if a Buick dealer installed a .040 engine in a customers car, they would use the same manifolds. Ben