My C30 came without the rear shock option and I am considering installing a set. There are no frame brackets for the upper mount of shocks which leaves me with a clean slate to build the brackets. This also leaves me with quite a few options and hoping to discover the best possible. For instance, Monroe says the stock rear shock on the 1970 C30 is
http://www.monroe.com/en-US/e-catalog/34803 which is 21 inches extended and 13.25 compressed.
My own reasoning is that a shock should be installed preloaded approximately 1/3 of it's travel distance. My reasoning for that is the trucks shocks are far more likely to compress with normal driving than to extend. Mass drops due to gravity quite easily whereas lifting that mass under the same normal driving conditions requires some considerable input of energy.
Going over a quick dip in the road may take some weight off the suspension (extend the shock) but the rate at which that happens is mild compared to the compression force as the vehicle/road level off.
Given that, I believe one would want to allow more travel to compress a shock absorber than to extend it (on normal road/driving conditions).
1/3 up travel and 2/3 down travel is just a rough rule of thumb that I would start with to get a shock length. That being the case, the Monroe shock would be installed under some compression to a length of 18.4 inches center to center.
Measuring that 18+ inches to somewhere along the center of my frame (an area I would presume to be for the upper mount), would place the shock at a front to rear position than an upward and downward position. That I think will reduce the effectiveness of the shock. I do have a lower mount point on my axle so I measure from that lower mount and try to determine where to best locate the upper mount bracket.
I'm thinking that a shorter shock may be better just to achieve a more upright position after install.
Is there an ideal angle for shock installation? Is there something I'm missing on this?