An X-Ray source is fine to use. In fact, it should give you a better radiograph.

Different voltages are applied to an x-ray tube to meet the demands of various classes of radiographic work. An x-ray machine for your application would be in the 150kV to 220kV range.

Agreed. X-ray can give a much prettier picture of weld. Assuming you really want to see what is in there (because x-ray will show everything much cleaner and clearer). We did some of x-ray in the lab I used to work for of material like that. It takes longer with x-ray than a strong gama ray source, so given a strong gamma ray source and many pieces to radiograph, gamma ray is the more economical and efficient way to perform the service so is used more often.

I did not think to mention that for radiography, you should use a 3" wide backing bar on your 1" plates. The additional width keeps the projected images of edges of the bars (and any tack welds) outside of the area of interest of the weld.