PlayGround

Head to head straddle mount (or center mount or head steady):
There is only the one upper motor mount assembly from 57-76 consisting of a support brace between the heads and a cross mount from there to the frame.
The support brace (straddle mount) is engineered to deal with any production tolerances and most owner created misalignments and is the first thing to assemble on top. ^{1) 2)}

The pics below show how the parts catalogs and at least the -64 FSM show to install the washers for the straddle mount.
Reportedly, installing the washers under the bolt head as shown can result in cracking the head at the threaded hole for the straddle mount bolt.
See Dr Dick's Top Mount Installation Instructions below for top motor mount instructions.

³⁾

There are three straddle mounts from 1957-1985;

• 1957-E1971 (factory fitted with the -57 head castings).
  These were straight along the edge that faces the rocker boxes.
  You shouldn't use this mount on any other head casting with out checking for clearance.
• L1971-1976 factory fitment.
  These were shaped like “Ben Franklin's eye glasses”.
  This part can be used on all 57-76 no matter what heads you are running.
• 1977-1985. All 3 holes are slotted.
  This should not be used on 76< frames.

1957-E1971 straddle mount 4)	L1971-1976 straddle mount 5)

⁶⁾

1. Assemble the straddle to the heads:
   • A 1/8“ thick 1” dia. washer goes BETWEEN each head and the bottom straddle mount (1 on each end of the straddle mount).
     Then the bolts (7/16“-20 X 7/8” bolts 5/8“ hex size) get put in with no washers on the bolts (between the bolt heads and straddle).
     The head is machined where this washer sits. The washer raises the strap above any unmachined nubs around the washer seat. ⁷⁾
     It keeps interference at bay. If you look at the different strap profiles, you see the 1000 mount is also relived for nub clearance.
     The 900 strap is not relived and when you see heads that are broke at the tapped hole, you will see most are 900.
     Busting the mount lugs is because the washers are put on top smashing the strap into the nubs and splitting the head.
   • No washer between the bolt and the top of straddle mount. Both the versions of straddle mounts get assembled the same.
2. The cross mount;
   • Gets placed under the straddle mount (all years)
   • Gets bolted with no washers (58-64).
   • 57 gets the choke mount placed under the cross mount.
   • 65-76 uses the stud in the horn mount instead of the bolt.
   • 66 gets the choke lever assembly added to top side of straddle.
   • Install Flexloc or Nyloc nut.
3. The cross mount to frame;
   • You will notice there is a giant space between the frame and the cross mount.
   • The cross mount and the frame surfaces will be parallel. If they aren't then something is amiss.
   • Select the correct pair of washers to fill 'your' space between the cross mount and the frame. This will take two of the three washers.
   • Take the remaining washer and slip it over the bolt. Both get slid in from the bottom, thru the cross mount, washer stack, and frame bracket with the nut on top.
   • At this point you got:
     • Three 1-1/8” OD washers (two thick, .160“) and (one thin, .090”/.110“ depending on what year).
     • A Flexloc or Nyloc nut
     • A 7/16”x20 bolt, either 13/16“ hex head 57-E72 (this bolt may be crossed drilled for a cotter pin/safety wire for R model use), or 5/8” hex L72-76.
       This bolt eventually goes in from bottom with either a castle nut or nyloc depending on year.
       • The bolt with the 13/16“ thin hex got 1 of the 2 thicker washers slipped on then these 2 items went from the bottom thru the cross mount.
         Then the other thick wash, then thin washer (these are slid between the top of cross mount and the bottom of the frame mount), then frame mount.
       • 73-up used a regular 7/16”x20 bolt, not 13/16“ hex, instead.
       • Now the nut on top.