I think the best advice anyone can give you is to experiment. I've used at least 3 different methods of using 4:3 footage in a 16:9 project in the past for a variety of reasons and I assume you want to avoid any distortion.

If the video quality and framing allows it my preferred method is to re-size the clip so that it fills the width of the screen. I lose the top and bottom that way.

If it's poorer quality or I need to keep the whole frame then I choose a suitable backdrop and centre the footage on the screen. You see the same effect a lot on e.g. "You've been framed" when they show footage from phones that have been held upright.

With really poor original footage I have shrunk it and played it over a suitable background. My favourite one is of a lounge with a TV in it and I have it so that it looks as if it is playing on the TV.

Hi, Tim,
Bringing new life and look to 4:3 video can be done several ways.
Start PD in 16:9 mode and place your old video in a timeline.
Right click the video and a menu will appear. Im not at my machine at the moment so I cannot recall the exact item to click on in that menu but there are 2 ways to stretch the video to wide screen, one is simply stretch, the other is CLPV, that method leaves the center of the video as it is and only stretches the perimeter, it often works quite well to simulate widescreen without making people in the video look like a herd of fatties.
The third method is to place your old video in a timeline as it is and use a background to fill the empty space where you would see the black bars. You see this method on TV sometimes when they use vertical phone footage.
Ok, thats for aspect ratio.
For general appearance to clean up old video click on a clip in the timeline and a FIX ENHANCE button will show up.
Use the sliders and such to get the look of your video as you wish. Denoise is quite adept at getting rid of grainy looks, and just experiment with the other parameters, color, lighting, etc. Keep in mind the more alterations you make the longer it takes to process, it can really strain your machine sometimes.
Good luck,
Barry HP Envy Phoenix/4thGen i7-4770(4@3.4GHz~turbo>3.9)
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Thanks for the advice guys, most appreciated. Would like to enhance the old videos & make them look reasonable on latest tvs etc. Does the fix enhance do an auto enchance to save fiddling about, or is an idea to folk out for colourdirector.