I would remeasure if I was you. No clamp was made in 29.4mm - only seat posts. There are three standard sizes for seat post clamps and these correspond exactly with the 3 sizes of front mech: 28.6, 31.8 and 34.9mm - ie if you front mech is a 28.6mm then your seat clamp will be 28.6mm as your seat tube will be a similar size all the way along. The only odd one is the 30mm clamp that is used on some carbon frames.
IIRC most Sunns used a 26.4mm seatpost and a 28.6mm front mech (the steel ones did at least) so the clamp should be a 28.6mm. I think you'll be searching for a very long time to find a clamp in 29.4mm