Torsion Bar Question

[GUMM B]

I've got a new friend with a 1971 Plymouth Satellite Sebring Plus that came with a 383 2bbl, half vinyl top, power windows, power breaks, power steering, ac, etc. I don't know if this means it came with heavy duty torsion springs or not. He has swapped in a 440 so now he's putting out 400+hp from his 8-1/4" 2:76:1 rear with aftermarket locker, all in his daily driver. He's a young kid so he intends on using it. I have a set of HD torsion bars (778 and 779) that I might sell to him. I wanted to know what the appropriate torsion bar would be for his setup. There are so many different bars in the parts catalog. Do we have the suspension experts here?

 

[Kern Dog]

Any stock bar is going to provide a ride that is softer than how many cars ride today.
The standards of what we grow accustomed to changed. Cars today do ride firmer and handle better but you can upgrade.
Even the biggest factory B body bar is too soft, in my opinion. If I recall, the .94 Hemi bars were the thickest ever.
PST/Kanter has the popular 1.03 size. For years I ran Mopar Performance 1.0 bars. Now I have 1.15s from the former XV Motorsports.
I have the stock .88 torsion bars in this:

....and while it handles okay, it feels nowhere near as solid and connected as this with the 1.15s:

 

[YY1]

I believe six cyl cars got a the smallest bars.
Small blocks were next.
"HP" small blocks next
Not sure if non-HP big blocks got the same bars as HP small blocks.
Then HP big blocks.

So, your car already has "big block" sized bars.

As stated above, HP bars is a potential upgrade, with an even larger aftermarket bar a step after that.

Does it have a sway bar?

FYI- higher end cars started getting rear sway bars in 72.
Almost all B body cars got them in 75-79 (but had "ISO" mounts with larger holes).
Those can be made to work, fairly easily on a 71.