Little help, C4 IRS

[Deabt54]

So I'm asking for a little help from those of you that have used a C4 IRS. I've got a 54 5 window with C4 IFS using a Progressive cross member. I'm r getting ready to install the rear, I'm going to set up the brackets myself and op out of using a Flatout or Progressive kit. Two things I hope someone can help me with.
1. I've got the distance between centers front and rear for the wheel base. Does anyone know the distance from the batwing like bracket to the axle center line of the third member. I figure if know that distance I could just measure back from the center line on the frame and mount the brackets. Make scene?
2.I'm going to run Aldan coil-overs, so this is out to you guys that are useing them what size and what spring rate are you using? And how much sage did you allow for?
Will I hope that all makes sence its kind of late.
I'll be posting more pics soon. Thanks in advance.
http://www.67-72chevytrucks.com/vboa...d.php?t=419646