Not sure if this picture is any good

 

I think the rust factor begins with the quality of steel used in the frame.
and ends with the way the parts are welded on.
If you're referring to the parts numbered 13 and 14 in your pic, I would think that they can be butt welded top and bottom. I guess they are attached with rosettes, or spots? that would allow water to get between the mating surfaces.
You'll need to do something about the tension, and compression flanges though.

 

Yep, that's it, parts 13 & 14

They're butt welded front and back and (I think it's called) lap welded top and bottom as the top and bottom have a lip which extends onto the main frame

The outrigger is a box section, but has holes running through it for the fuel lines

The holes obviously trap water, and sits in between the outrigger and mainframe where it rots through

I don't have access to my car to take pictures, but I'll see if I've got any on my phone which I can post

 

 

 

is the rest of the frame ok ?
that looks pretty bad

 

The rest is just surface rust

Luckily that's not the chassis I'll be using, it's a spare I have, but it gives a good idea of what I need to prepare against

 

Looks to me like it was untreaded metal, the front tires throw road salt into and onto the steel. Rust wins.

I would suggest cleaning all the rust you can get off the frame by blasting and or dipping. Then SPI epoxy to the interior areas of that outrigger, then weld shut those round holes then SPI epoxz the complete frame

 

I was thinking of powdercoating the chassis and all the running gear - what do you think?

Could I get a finish at least as good as the red car shown above if I went with pc?

If I do choose pc'ing should I strip the inside of the outrigger myself prior to welding it in place?

I guess if I got it blasted there would be media stuck in the box section which could be a problem

Although, if I did weld up the holes in the outrigger first that shouldn't be an issue?.....weld up the holes, epoxy inside the box section, weld outriggers to mainframe, then get blasted and pc'd??

 

Here are some images of the outriggers

I'll take some photos when I get to my car over the weekend

Note, the side facing downwards in the image will face forwards when fitted to the chassis

You can see only half of the outrigger is boxed in and there are holes at the top, bottom, front and back faces of the outrigger

Should I weld up these holes and reroute the fuel and brake lines, or should I fit grommets/bungs into the holes?

Thanks