a block of wood, cut with a slot in one side the width of the pedal arm, will work as a spacer between the firewall and the pedal arm on the engine side of the firewall. have somebody step on the clutch so it is at the floor, measure the dimension between the floor and the pedal arm, make the block with that dimension. next measure the dimension from the seat base to the clutch pedal when it is on the floor and make up another piece of wood to fill that void. step on the clutch, slip in the long peice of wood to hold the pedal down. then slip in under the hood and put the short piece in. then the long piece can be removed. this will keep the pedal close to the floor while it is stored without worry that the block is gonna damage something inside the truck, upholstery and whatnot. when tthe truck is started and the clutch pedal is stepped on the little block of wood should fall out and be retrievable for the next use.
or just use the longer piece and leave it stuck between the seat base and the pedal, forget about the short piece and the associated dirty work, lol.
there are shop tools made for this use for mechanics to use. they are more or less a rod that is shaped to hold over the pedal well and have an adjustable part for length. they are not usually made to hold the weight of a clutch pedal but you could fab one up that would hold. some have a mechanism like a caulking gun to adjust and hold the length. wheel alignment, brake light checks etc was what they were intended for.
https://www.cromwell.co.th/shop/auto.../p/KEN5031910K