1940 Ford transmission help

I have a 40 Ford Trans that is behind a 283 chev. The problem i have is when it warms up after about 15 mins of driving i can not select 1st gear with out alot of grinding. I have changed the oil to 85/140 then 140 but there is still a problem, just after any other suggestions before the motor and gearbox has to come out
Thanks Marcus

 

Check the clutch adjustment; you may not be getting full release.

 

My guess is that it is a clutch issue, not fully disengaging once the disk/pressure plate/flywheel heat up and expand, maybe a broken finger or spring

Of course 1st gear has no synchros so it will be worse than 2nd or 3rd...