A couple of thoughts come to mind - if your 58 is much better, something may still not be right with your 61, since they are fundamentally the same. First thing I'd check is to make sure the wheel cylinders are the correct size front and rear. I don't remember the specifics, but I believe either the front or rear are larger than the other. If they aren't correct that could cause a problem. Also, it might pay to check the shoes to make sure the primary and secondary shoes are installed in the correct orientation. The primary (short lining) goes forward. You might also check the drums for blue spots. If the brakes have been overheated, they develop blue spots which indicates the drums have hard spots and don't grip as well as a smooth freshly turned drum. Also check all three flex hoses. They tend to deteriorate from the inside out, so if there is checking on the outside, they are probably shot and swelling when pressure is applied, which in turn reduces braking effectiveness.
If all that is tip top, a booster will definitely reduce pedal pressure. I ran the stock manual brakes in my 1960 for several years, then swapped in a 75 front suspension with disc brakes. I also ran the disc brakes with a manual master cylinder for a year or so, but it got to the point where a couple of urgent stops turned my chronic back pain into severe back pain. If I hadn't added a booster I wouldn't be driving it any longer.
