What size torsion bars for a 383 in a 75 Dart?

I just put in 1.14" torsion bars from Sway-A-Way as I do track day/autocross type stuff, with an aluminum-headed small block up front the ride is still quite nice; not any more harsh than a typical modern car. It is a tad bouncy because I haven't put in proper shocks to match but it's really not all that different from when I had 1.0" bars before or the stock 0.8x" whatever 318 ones. In my experience the ride quality doesn't change nearly as much as you'd expect with stiffer bars.

With that said and assuming we're talking about the car in your avatar I'd go for something in the 1.0-1.08" range, I'm also a believer there's no reason not to run at least 1" with modern tires on these cars. It'll be a nice improvement from stock, IMO factory bar sizes are all way too small and it's because of the tires being designed for back then.

Unless you do want to drag race it then smaller bars will actually help weight transfer on launch and make total sense.