1st. is probably the only gear that's not synchronized. Some transmissions also use that gear for reverse. Try adjusting the clutch for a little less free pedal at the top of the stroke. As long as the throwout bearing isn't riding on the clutch fingers all the time, you're good to go. A little less than 1" free play is plenty. It sounds like the warmed-up clutch disc is dragging on the flywheel and/or pressure plate a little.
Until you manage to get the clutch adjustment process figured out, here's a work-around. When the transmission is in any gear, everything, including the clutch disc, is stopped because the driveshaft isn't moving. Try selecting another gear, maybe second, and then very quickly moving the shifter into first. There might be a slight clash of teeth, but it will be very brief because the countergear won't be able to spin up. That's a trick we used to use on the big rigs with divorced transmissions, like a 4 X 3, or maybe a 5 X 4. Moving 40 tons of truck and cargo around by stirring two shifters without using the clutch except for starting and stopping took some practice, and sometimes we earned the title of "gearjammer"!
Jerry
