I have a '68 SS 396, 4spd (original). Now with SB350. Plan to put big block back in, in the future. Need to replace exaust syst. Would like to install 3" pipe and mufflers, and reduce to 2 1/2 " before manifold. Is this a feasible approuch? Thanks for any replies.

Not sure what you're getting at. Why not put on headers? They generally have 3" collectors so you would not need a reducer to fit 3" tail pipes.

I agree, seems strange, is there any reason for this arrangement?

Will the headers for the small block also fit the big block when that is installed? I don't want to buy a second set of headers.

no,completely different