Got you. I did under head lengths. Looks like we have same bolts. The only place there is room for extra length is at the firewall and core support. Thickness and density of mounts and cushions also dictate bolt length as well. I find most after market mounts are much denser than originals. Like Hockey pucks.
These are original body mount bolts. 71 Skylark. I don't have these particular bolts now, traded them along with some remaining 71 Skylark parts to a friend, but can provide his contact info.
Just finished chasing this to ground on my project so I thought I would share my findings. At the core support, bolt GM 1386924 is a 7/16 hex bolt (although mine was eroded so bad I couldn't tell if it was originally a flange or just a hex bolt), my Flint original measured 3-3/8". I'm using AMK B-11631 (3-1/4" long) although it appears Inline INL13471 is equivalent. AMK is gray phosphate and has the "A" head marking. The thick washer GM 3700338 is AMK B-13021. The hex nut is AMK B-12796. At the cowl (#1 and 2), GM 7/16 hex flange bolt 3902232 measured 2-7/8". Inline's INL13472 measures 2-3/4" and has an "O" marking. For Flint built cars, the nut used is a 7/16 hex flange nut. I haven't found an exact reproduction. Cars built elsewhere had the 4-sided square nut. I'm using a 7/16 hex flange locknut, slightly different but is gray phosphate. Under the door at #3, GM flange bolt 3902231 measure 2-3/8". AMK's equivalent B-11629 is slightly shorter than Inline's and measures just under 2-5/8". AMK is showing one bolt to replace both 3902231 and 3902232, and at 2-5/8" is in between the two (2-3/8 and 2-7/8). For 6 and 7, GM 3902232 is replaced by either the AMK B-11629 or Inline INL13472 My original bolts had head markings that were a mix, some were an "E" and some were an "F" inside a circle
Not satisifed with the 4 nuts at #1 and 2 (cowl), I found a better solution. The 67-68 Mustang Shelby export cowl brace is secured with a similar fastener. It is AMK B-12906, a non-locking flange hex nut in grey phosphate, with serrations. It differs slightly from original. If you don't have the $30 minimum order for AMK, it is also available thru Mustang vendors as AMK F-205, a kit with the 4 nuts and some bolts.
Flint only used a non flange bolt at the core support so flange is not corroded. The head was bigger than the standard head that would normally be on a 7/16" bolt. The bolt under the battery is usually in rough shape. If you insist on these bolts you may need to find two from the passenger side of two different cars. No easy task.
