Thanks for the pointers to Car9! I don't really do enough to promote it. There were several inspirations that started me to work on that project and one of them was the bunch of tubing behind the aft bulkhead the driver sits against. Then one day I saw a picture of Modernbeat's car where all that tubing had been cut off and the car was still a roller! It was a live axle build - but that did explain to me why I wasn't sure what that tubing was doing. Using that stuff to "support" the roll bar made even less sense to me. I think the roll bar is holding the rear tubing there, not the other way around.
In addition the material around the diff which the rear suspension connects to appears to sort of cantilever everything off the short lower tubes which are connected to that bottom cross tube in mid-span?
I wanted to better understand all this and I had a hunch some changes might help things. I used a free FEA analysis tool called "Grape" that works on tubing or truss type structures. It's a much simpler problem than other FEA approaches that involve dividing the material up into little triangles. This model is available in the chassis stiffness and Car9 threads, so yo can play with it. There is also a Locost model in the chassis stiffness thread.
First thing I did was raise the upper chassis rail a bit. Our modern motors tend to be a bit taller and this increased frame stiffness by %25. For safety and proper bracing the upper rail continues around the back of the car at the top of the trunk. More than one person on this board has been hit from behind at a traffic light and my third trip to a track in a formula car I put it backwards into a cement wall. The lower rail also continues around back but it kicks up a few inches. These changes allowed very sturdy bracing for the roll bar.
You did fix one of the little issues on a Locost frame. There is a little jog in the tubing from the engine compartment to the transmission tunnel by the driver's gas pedal foot. With the load we were using to test torsional stiffness, this little jog was lighting up with over 70 KSI in bending. I assume the floor should be strongly welded here and not riveted! In Car9 I kept the stresses to around 12-15 KSI for the same loads.
Another difference over what you are showing is that I chose to use longer trailing arms and a reverse wishbone for the lower control arm and a simple radius rod for the upper arm. This was traditional on many cars of that era and offered a lot of simplicity. The trailing arms feed the fore and aft load into the side of the frame, which is very strong in that direction. The lateral loads are fed into the bulkhead behind the driver, which is also easy to make enormously strong. Again you could remove the rear tubing and the car would be fine. I t just seems so much simpler to me. Later I found pictures of a Super Seven delivered from the factory with IRS. They also chose this arrangement.
One thing is Car9 doesn't have a front roll hoop. I'd like to see one worked in actually, it just wasn't on my first builder's list. Extreme car should be used with the roll cage arrangement because those tubes are just baseball bats aimed at the passenger's heads. In a real accident your spine can stretch a great deal. Several inches in the back and also in the neck. I'm not sure there are good answers here. They sell special padding for this which is quite hard, but even then it may be expected you are wearing a helmet - maybe it's helmet protection...
See if you can find a short oil pan your pushrod motor is probably shorter than a modern 4 cylinder and that's the difficult dimension to package. No problem with your choice but will mention that a small block Ford is much smaller. Think through your power situation a bit, these are light cars and you don't have a lot of traction. Being able to break the back end loose at over 100 MPH - that's serious business. I was thinking on that on my way to that cement wall - remember it pretty clearly and it was 40 years ago...
There's lots of great info on this site and plenty of people willing to hep and encourage you. Have fun! Any questions, just ask.
Here's a couple of pictures of Tom's build. He's all grown up now though.