Look for rust that might go through, or pin holes. Around the rear shock and frame rail is where my 67 went. No leak but would do odd things like go to zero pressure in the 1/4 mile. Would just make it through the lights with the fuel in the bowl. Pressure at idle was OK.
Check the pickup sock filter. Be very careful when rmoving the pickup. The original senders are a bit fragile but the only ones that read correctly.
Side note: A few, very few, big block cars got a 3/8 pick and line.