1951 F4 loses power in 4th gear?

[corvette8n]

Sounds like you have more than one issue here, you need to trouble shoot one thing at a time. Put the truck in 4th gear rev up to about 1500 rpm let out the clutch slowly if truck stalls your clutch is not slipping. Grinding gears could be clutch not releasing or bad syncro gears in tranny. When you say it feels like it is losing power do you mean the motor is reving up and you are not going anywhere or the motor is slowing down?

Nice looking truck.