Be aware that as well as the distance between mounting holes, you need to get the height of the mount correct. If the rear mount is still present, this can be the reference (level between mounts), otherwise get the correct mount height.
On a friend's swingarm frame where the front mount had been cut off, the new mount was welded a bit too high, and the handlebar clamp nuts dug into the top of the tank on full lock. The mount had to be removed and remounted lower down. If there is only a slight difference, the steering lock can be reduced by building up the stops on the frame. I have used a bent bit of alloy strip secured by the steering damper plate bolt.
A mod to consider (I have done this with a swingarm tank, but it should be possible with rigid/plunger models) is to use car rubber exhaust mounts (I think I used MG ones as being the shortest I could find), which can be attached to the tank (possibly turn down the nuts to slip into the tank mount recesses), and just bolt the tank in place. It would help location of the tank if the bottom nuts are turned down partially to the diameter of the frame mount holes. You then have a completely rubber mounted tank, if it moves about too much, wrap some old tyre inner tube around the frame centre spine to limit the movement. You will need to check the steering lock for handlebar clamp nuts touching the tank, depending on the degree of contact this may be solved by the use of half or reduced hex nuts .