The method of construction (engineered trusses and glue-lam beams) used in the building of the house where Walker works and ultimately hangs himself were not used nor existed until the 1980s.
The first time Jim puts a needle in his arm in-front of Kristen, he never pushes the plunger in, when he unties the belt, you can clearly see a few inches of white plunger, if he had pushed it in, the plunger would not be sticking out. So therefore he did not "shoot up". He simply stuck the needle in his arm without injecting the contents.
There's a scene in which Raynor takes Kristen to a gravel pit. He separates a 6-pack of beer and twists the individual cans down into the dirt. Kristen empties a revolver at the cans, hitting four of them. Raynor retrieves the 2 remaining cans and gives one to Kristen. During their subsequent conversation, Kristen plays with the sta-tab on the can, except that the film is set in about 1972, and the sta-tabs didn't go into use until maybe 10 years later.