The biggest thing that effects the length of the front hairpins is the type of steering box you use. Personally, it's hard to beat a Corvair box. They are tough enough to stand up to the use it will get in a T and small enough to eliminate a lot of clearance problems. The Vega box has to be mounted toward the front of the frame and with most frames being built so low, ground clearance becomes a problem. I did a swap on a T last summer for just that reason.
With that said, you want to set up your hairpins so the steering box is right above the frame hairpin mount. That way both the hairpin and the drag link will swing in an identical arc, greatly reducing bump steer.
The rear hair pins should ideally have the frame mounts in line with the front u-joint of your drive shaft. Depending on the trans and wheelbase of the chassis, they can get to be very short. You can fudge a little on this length because of the slip yoke at the trans. Another option would be a 4 bar setup.
You really want to use 3/8'' stock for the axle hairpin mount. For the frame hairpin mount, if your chassis is low, you might want to go through the frame rail. In that case sleeve the frame rail with a piece of tubing. If the mount hangs below the frame rail, double up on the plates and use a piece of tubing to keep the bolt for the bushing stable. 1/4'' stock will be fine for these mounts.
Sorry, didn't mean to get so long winded.