It also depends on the hub you are using on the rear too. It's pretty hard to work out the dimensions you need - and with mine - I lucked out somewhat; I can say though that it's likely the 127mm spindle will be too long and you should be looking at the sizes around 113-120mm; If you fit a crank that takes two chainrings then this offers a fair amount of flexibility over where you fit the chainring and you should be able to get a pretty straight chainline. [on my twenty the chainring is bolted backwards to the inner side of the crank - as if it were the second chainring] It's also worth noting that one of the real attractions of fitting a un72 with the phil rings is that you have some leeway in how 'centred' you fit the bb cartridge, as the p.w, rings can be screwed right inside the shell; this means you can 'dial in' - if need be - one or two crucial millimetres of offset to the cartridge to get the drive chain straight.
If the worst comes to the worst - you'll have to get a different crank and return/ebay the one that doesn't give you a straight chain. Which isn't a biggie, and all part of the fun/learning curve
I'm using Redline 165mm BMX cranks on my R20 with a 48t ring and a 127mm UN72, which as LittlePixel points out is way too long, so I've had to use long bolts and spaces to achieve a straight chainline.
If you are using the SA 3 spd hub the 115mm spindle should be right, and depending on how strong you are....get a bigger chainring than the standard 46t.
I'm interested in this subject also. I'm thinking that if I get a 20, then I'll take the BB apart an hopefully find an cotterless axle that's the right length to fit with the original cups and BBs. I've got enough old axles hanging around.
But then I got to wondering, is it just axle length that is a potential problem (that is, regarding aligning the chainring), or is it the spacing of the BB runs on the axle that is different?
i find that modern suntour MTB cranks sit very close to the BB cup so need a longer spindle.
old Shimano cranks seem to take short axles
I like the old style straight arm cranks, dont like these bendy ones that are available now.
having said that. I want a set of Shimano road triple cranks, from a few years ago. As they have big rings to enable the 20 to get some speed up. But also have a granny ring. And would work with indexed shifters. But I was out bid on a few cranks on ebay. So gave up on them