mixing media HDV 1080p 30fps with SDV 720 29.97 converting in QT keeping quality

Of course this is nothing new to us editors...

I will be working in FCP to mix media HDV 1080p 30fps with SDV 720 29.97. Final Project will be SDV 720 x 480. I thought that by converting the HDV in QT to match the SDV would keep the beautiful quality even though it's smaller in size, but I was wrong. My idea is to convert the files make it easier to work with in the FCP sequence and not have to render all the time.

Question: Does anyone know how to keep the HDV quality and make the files all the same size? Or do you have any suggestions to make it the best quality possible?

thanks,
Lauren

What's happening to the HDV when you convert it, and what exactly are you converting it to?

ProRes is a good intermediate codec.