Hi

General question.

What would be the best all round spec for a performance exhaust for a xflow?

1660cc, Twin 40 delortto's, gt head and BCF3 cam

You need to work out the lengths of your runners using your valve timing and make it to suit.
4-2-1 for fast road, 4-1 for track.
All the info is in the book; 'Rebuilding and tuning Ford's Kent Engine', by Peter & Valerie Wallage.