For a 3100 the axle center line is 15.75 inches in front of the dash. The dash is 43.24 inches from the front end of the frame. Therefore the center line of the axle is 27.49 inches from the end of the frame. Another reference is from the center line of the cross member and spring hanger for the front of the rear leaf spring. This dimension is 71.97 to the front of dash, plus 15.75 to the axle center line. This is 87.72 inches from the cross member center line to the axle center line. And anothr reference is from the bell housing cross member at the center line of the rear motor mount holes. This is 7.57 behind the front of the dash plus 15.75 in front of the dash to the axle, this is 23.32 inches.

These are the dims from the assembly manual, but as 48BT said, most will use the sheet metal to locate the front and rear wheels when making these modifications.

I hope this helps.

54 Chevy build pics.