Well its a 1976 Silverado, im pretty sure its a Silverado and not a custom deluxe or any other model, although my speedometer reads 100mph but all other trucks read 85 as max what does this mean? Its factory speedo. Also how can l check when it was made and if its the original engine? Thanks for your help in Advance!
ok it has GM Goodwrench on engine, where exactly would l find the VIN?

Your vin holds the key to all the you seek. You can go to a dealership and they can run your vin for you. You could also search for a vin decoder on any search engine and it will tell you options color what production number what plant and maybe what day. your engine should have a stamp some place visible on the block.

VIN is found in the lower corner of the dashboard at the bottom edge of the windsheild.

A ''Goodwrench'' engine is a sure key that its probably not the original engine.

The vin can also be found on the drivers side door jamb. The door jamb label will also tell u the month & year the truck was made (such as 06/1976). The vin will also be on the title. The early versions of these trucks (1973 to around 1980) have 100 mph speedometers. The 85 mph versions were used after the national speed limit was dropped to 55 & have 55 highlighted in orange. The Goodwrench engine was bought from a GM dealer within the last 10 years or so. Before that the replacement engines were called ''Target Master.'' As far as the original engine goes, find the 3rd character of the vin:
D = 250 cubic inch in-line 6 cylinder / 1barrel carb / 105 hp
T = 292 cubic inch in-line 6 cylinder/ 1 barrel carb / 120 hp
V = 350 cubic inch V8 / 2 barrel carb / 145 hp (n/a in CA)
L = 350 cubic inch V8 / 4 barrel carb / 160 hp
U = 400 cubic inch V8 / 4 barrel carb / 175 hp
S = 454 cubic inch V8 / 4 barrel carb / 240 hp
Some of these horsepower ratings vary by application & whether or not the truck was to be sold in California.

Also, Chevy VINs before 1979 do not work on most modern vin decoders, as there was no government standard for their format before then. To find this info look for books or websites specializing in older chevy/gmc trucks. Most dealers do not have info readily available for vehicles over 10-15 years old.