I would caution you against this at least as far as doing the final weld in until you get the cab in place. Unless you do not have the cab at this time I would suggest placing the cab in place temporarily to get the location set correctly. There are just too many variables such as engine installed height and firewall clearances.
I am unable to give you the dimens you ask for as I have a g body subframe on mine. I can run a tape from the rear k member to a point on the engine but I would not do a firm install on that alone.
