Searching For A Dataset For NFL Salaries Going Back As Far As 1967

I know this may be a pipe dream, but I’ve been searching for salary information on 10-15 NFL players, most of which played in the ’90s, as far back as 1967. Does anyone have any idea as to where I could find this? nflfastr only dates back to 1999, sportsdata.io only goes back a few years

For reference, I am doing a draft analysis and I am trying to compare previous draft pick trades. I have a “draft pick value” chart, which gives me a number value for each pick in the draft. I want to compare each side of the trade and say if it were a “good deal” or a “bad deal” based on the total value from each side. This works fine when there are only picks being traded, but when there are players involved, I am having a hard time objectively comparing each side. My thought is that the first overall pick is paid a certain amount of money when they are drafted, so I can find the value of a player by giving them the value of the first overall pick multiplied by how many first round salaries are within that players contract. (ex. pick 1.01 is worth 3000 and is paid $7M/year, player is paid $14M per year and worth 6000 because 3000*2=6000). Any ideas on a different way to try this would help too.