There's no standard "34 ton" designation for a Chevy truck. Chevrolet didn't produce a 34-ton 2-wheel drive truck in 1987 (or ever). The weight capacity of trucks is typically expressed in tons (or more commonly, GVWR - Gross Vehicle Weight Rating) but it wouldn't be part of the model designation. The 8-lug wheel pattern also varied depending on the *actual* model of the truck (e.g., C60, C70, C80).
To find the lug pattern, you need to identify the precise model of your 1987 Chevrolet truck. This will likely be found on a sticker inside the driver's side door jamb or on the vehicle's title/registration. Once you know the exact model, you can then find the correct lug pattern specification in a repair manual or online resources for that specific truck.
