Any angle will do, as long as it isn't flat. I built a drag car for a guy in '97 that is still doing the Super Gas thing and about 100 passes per summer and the only driveshaft tune up it gets is an annual replacement of the U-Joints-----The car sits very low with the transmission tailshaft being 1" higher then the pinion and the pinion angle is a 3.5 degrees down. The rest of the suspension is nothing more then coil overs and ladder bars with an adjustable lower link that we use for a bit of "preload adjustment" to dial in for track conditions.....

You may have a slight vibration, but if there is any at all it will be somewhere between very slight and totally un noticeable..... Get someone good to build your shaft and balance it very, very close!!!!! A few more bucks spent on the build will save you a ton of grief and expensive problems later.....