1 pointI think this should do it... ffmpeg -y -i input.mp4 -vf "setpts=1.25*PTS" -r 30 output.mp4 Changing the -r value to match your needed framerate
1 pointYou're overcomplicating things. You have video that has 300 frames. You need it to last 60 seconds. Necessary frame rate is 300/60=5 frames per second. Who would want to write a specific tool that does one division operation? Certainly not me.
1 pointSome free books can be had from Fusion Retro Books... The Story of the Commodore Amiga in Pixels The Story of US Gold The History of Ocean Software The Story of the Oliver Twins The Story of the ZX Spectrum in Pixels - VOLUME 1 The Little Book of ZX Spectrum Games The Story of the Commodore 64 in Pixels Ted.