The focal length does not matter at all.
What matters is how far away the primary mirror is from the secondary mirror.
If the focuser does not have enough inward travel to reach focus, the primary mirror has to be raised so that it is closer to the secondary. This can sometimes be done with the existing collimation screws, simply by running them just short of fully clockwise, but if there is not enough travel, longer screws could be a solution.
If the primary mirror cannot move closer to the secondary, then the next possibility is moving the secondary to the primary.. If the scope has truss polls, they can be shortened as required.
The last option is to use a lower profile focuser like the Kineoptics helical focusers.
Now, here is my fear for you.... I would guess that you did not use a Newtonian design program because had you done this, you likely would not be having this problem, because the program would have asked the focuser height. The fear is that if you have to move the primary mirror too far far up, depending on the size of the secondary, you could cause aperture loss. For example, if you selected a very small secondary mirror with a very small fully illuminated circle, even a small decrease in the primary/secondary spacing could cause aperture loss.
From your description though, your problem is not a "focal length" problem, it is a primary to secondary spacing problem.
If your secondary is particularly large (25% or 30%) this fear is probably not in play, but if you started with a very small secondary, my advice would be to find a Newtonian program and make sure that when you move the primary up, you are still going to have full aperture.
Edited by Eddgie, 21 July 2019 - 12:58 PM.