I have a booster from a 67 Chevelle (4 piston calipers) and a master cylinder from a 69 Chevelle (single piston calipers), but I am missing the pushrod that fits between the two.

Can I just cut down piece of steel rod and round off the ends? I'm envisioning that's what the original rooked like. Sound OK?

I would look for a push rod for a 69 chevelle. When I converted my '67 I used the master cylinder from a '70 camaro. I found that there was a difference of how much of the master sat into the booster. If you can't find one find a brake store( Brake and friction material only) And see if they have an adjustable pushrod. Be very careful on your setting on an adjustable rod, too long the brakes could lock up, too short and you'll have too much pedal play.