Like Ron said, more details are needed. How much of the system did you replace/overhaul? Dry system? 32vld already covered one question, bench bleeding the master cylinder? Using proper bleeding procedure (which may be different from the '61 procedure due the change in parts)? How are you bleeding the system? Pumping the pedal? Speed bleeders? Vacuum bleeding? Pressure bleeding? Have the system been checked for leaks? Brake shoes properly adjusted?
That said, if you have bench bleed the master cylinder, along with at least the line that would need changed to make the dual circuit system work, you have at least a 50% dry system. That is going to take longer to get all the air out, especially if you are manually bleeding the system. Without seeing just how the lines are routed, here's what I would suggest; With the engine off, rebleed the master cylinder. That can be done with it on the truck, no need to remove it, just disconnect the lines, install the bench bleed kit back up, and have a helper pump the pedal. Reconnect the lines and tighten until the are almost seated, then have the helper push the pedal down, then tighten the lines. Next, start at the wheel with the LONGEST line (you'll have to figure it out according to how the lines are routed), bleed that wheel first. Bleed it as many times as needed, checking the master cylinder after about every 3 times to make sure it is full, until no air comes out. Repeat on the next longest line until all 4 wheels are done. Check for pedal feel, both with the engine off and started, then go around them again bleeding at least 2-3 times per wheel. Recheck pedal feel again, bot with the engine off and started. That procedure should get you a good pedal.
