This is how i had to do mine on a set of B1s. Bring piston to TDC. Use an ice pick or short skinny screwdriver to hold valves close to seat. Remove springs, clean valve stems and wrap masking tape around valves. Try and keep tape as close as possible to seals. Also if the seals are good they should keep valves from dropping out.

To reinstall just remove tape and push down on ice pick and it will lift valve up far enough to get locks and retainer on. I also put grease where stem meets the seals, it was sticky enough to keep valve from moving as i compressed springs.