What year and model is your truck? Knowing that could make a difference.
I'm going to disagree with the approach SwingDaddy suggested. While the '54/'55 1st cross member with a later bell housing is a good approach when using the original motor I think in a case like yours there is a better way.
The 250 does not front mount, it needs to be side mounted. If you go with a V-8 type side mount cross member in the front and a similar transmission mount at the rear, you're good to go with that integral bell housing and its hydraulic clutch support.
I've not done it, but that's the way I'd approach it if I were doing this.
From there the question becomes pedal and cylinders as you suggest.
Are you planning to run power brakes? Dual master cylinder for the brakes?
The usual approaches to floor mounted pedals with dual master and a power brake cylinder make the area under the floor somewhat crowded. But you could still mount a clutch master on the frame ahead of the pedal pivot. You'd need to stay away from the exhaust but otherwise it should be a straight forward project.
