I suggest you print out the geometry specs of the Alien Frame, take them to a local shop, and sit on bikes. Find a bike that has an EFFECTIVE top tube length (because so many bikes have compact geo now) that works for you, and choose the Alien which has a similar effective top tube length.
I know plenty people who based their bike simply on stand over/inseam and they ended up with a bike that was actually stretched out too far, or had them too crunched up.
For example, I usually ride a 50-51 cm seat tube bike as long as it has the common 52ish (520mm) top tube length with a 90mm stem. I could never ride a Leader 51 because their 51 is made with a 54.6cm top tube length. Yea I could run a 70mm stem as opposed to 90, but that's a band aid fix. See what I mean?
Hope I don't sound condescending or rude. Just trynna explain how seat tube length isn't the by all end all equation to sizing an off the shelf frame.
Have you played with any fit calculators to see what they have to say?
Tiemeyer Cycles, Wrench Science, and Competitive Cyclist all have calculators that will give you some BALLPARK ranges that shouldn't be taken as gospel, just for comparison. I haven't used the CC fit calc but Tiemeyer and Wrench Science, incorporate torso and arm length in addition to leg inseam.
I am pretty average in terms of body proportions so the bike I have now actually falls into what both wrench science and tiemeyer recommend for someone with my measurements. I was surprised it was fairly close