Nice build, the dual spindle will be nice. Would be nice if you could vary the distance between spindles on the gantry huh? Depending on how much you use you may want to take the guy up on going with a VFD spinidle (I got a 4kw (5hp) from Ebay for about $700 for spindle + VFD, but buy them separately & it's less - it's really nice, but added cost) I used a porter cable 690 (the 1.25hp) for probably 3 years or so, cut tons of plywood & stuff no problem.
I use the pair of 1200's on the X (driving the gantry (my gantry weighs about 350 or 400# with the Y & Z carriages & A/C head & all)). They will drive that thing at 360ipm no problem. I'm using a 1200ozin on the Y carriage, & an 1840 on the Z,A, & C. I have a 2550 on the Rotary. These will all turn about 400-600 rpm no problem. I don't know where they're getting bigger is slower, they're not. I moved to a larger (1840) on the Z axis, but I use rack-n-pinion to drive it like the X & Y so I can keep speeds for carving, plus my Z + A/C axes weigh about 150#. With your 72V power supply I would not worry about being able to get the speeds, just make sure the ps has enough amps to run the motors the way you wire them - bipolar parallel pulls twice amps of series.
Also, I ran those 1200oz motors on a 24V power supply for years & was able to get speeds of 360ipm pushing about a 200# gantry/Y/Z with no problem. I now push the 1200's with a 54V power supply & the rest of the motors I'm using an AC voltage dirven Stepper driver from HomeshopCnc. Evidently they vary the output voltage & are a cost savings over buying a PSU + the Geckos. They drive the steppes really smoothly & will push even the 1840's to 400rpm & the 2550 to nearly that with about an 80# lathe chuck.
calvinoArchitect - YouTube
nice work!
Cheers
-Calvino