Somewhere I have a table that shows the measurements of various cars and trucks of the era that is sort of a poor man's Throttle Lever Checking Gauge. I'm not sure if it cover's '56 Chevy trucks (would also need to know 6 or V8 - they are different on GMCs - I don't know about Chevys), but I can look for it later this afternoon. It would at least get you in the ballpark.

'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.