My main concern is that all the elements are matched/ compatible to create a smooth/ reliable running/ better performance than stock daily driver.

Pardon me for stating the obvious, but step one would be to have the block and head types matched/compatible. This is a lot more of a problem than the exhaust size.

(I am now going to ignore people who will no doubt say that using a B18 head with a B20 bore, or vice versa, is just great. You're welcome to your own opinions.)

You can bore the block you have, use a B20 block, or go back to a B18 head. A properly tuned B18 isn't feeble at all, you just have to rev a little higher for essentially the same power as a B20. If the block has already been rebuilt, I'd go the B18 head route.

In any case, 2-1/4" is not too big for that manifold/downpipe with a D cam, regardless of the smaller displacement.