Measuring the spacing of the bolt holes thru the cross-sills should get you real close, assuming they're still visible and not rotted away. I didn't have that luxury, but went with new cross-sills and a wood kit. Your plan of adding to the outer boards sounds like a good approach.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
