Use the spacers and single bolts of the correct length.
This keeps the frame tubes together around the sandwiched engine cases. Very important! ¹⁾

Bolt lengths from parts catalogs (all have 3/8“x24 threads):

As you can see below, the lengths are far from being standard length and those lengths changed depending on what parts catalog you reference.
These are general guidelines for bolt sizes:
The front motor mount uses 3 different size bolt (lengths) for the 4 holes. ²⁾
The upper front and lower front holes (attaching to the frame) use the longest bolts.
The upper rear (through the case) uses the shortest.
The lower rear (through the case) uses the remaining middle length one.

If you use any other length bolts (in the case of the bolt going into the threaded engine plate at the top rear) use the 5-3/16” long bolt.
A longer bolt there will run into the generator mount and the lower one gets close to the oil pressure switch. ³⁾

• 1957-1975: ^{4) 5) 6)}
  • Top rear bolt (1), 5-3/16“ long (16235-52) grade 5 zinc.
    Part number changed to (16235-52A) in the -78B catalog (issued in 1989) but the length stayed the same in the book.
    Correct length is 3/8”x24x5-1/4“.
  • Bottom rear bolt (1), 5-7/16” long (16236-52) grade 5 zinc.
    Changed to 5-1/2“ long (4413) in 1973 and sold for 54-76 models from the -78B catalog (issued in 1989).
  • Front bolts (2), 6-13/16” long (16237-57) grade 5 zinc.
    '57 to '76 in the -78B catalog.
• 1976-1978: ^{7) 8) 9) 10)}
  • Top rear bolt (1), 5-3/16“ long (16235-52) grade 5 zinc.
    Changed to 5-1/4” long in -78A catalog (issued in 1981) for 77-78 models with the same part number for both lengths (16235-52).
    The correct part number is (16235-52A)
  • Bottom rear bolt (1), 5-1/2“ long (4413) grade 5 zinc.
  • Front bolts (2), 6-13/16” long (16237-57) grade 5 zinc.
• 1979: ^{11) 12) 13)}
  • Top rear bolt (1), 6-13/16“ long (16237-57) grade 5 zinc.
    Both the -79 and -79A parts books show a quantity of (3) for this size bolt (same size as the front two) instead of the 5-3/16” bolt.
    Changed in 1980 to 5-1/4“ long which is part # (16235-52A) and correct for 1979 also.
  • Bottom rear bolt (1), 5-1/2” long (4413) grade 5 zinc.
  • Front bolts (2), 6-13/16“ long (16237-57) grade 5 zinc.
• 1980-E1984: (all lengths sold for 79-85 models) ^{14) 15) 16) 17) 18) 19)}
  • Top rear bolt (1), 5-1/4” long (16235-52) grade 5 zinc.
    Shorter bolt with same part number as the longer bolt in 1978.
    Part number changed to (16235-52A) in the -85A catalog (issued in 1990) but the length stayed the same in the book.
    The correct part number is (16235-52A) at 5-1/4“ long.
  • Bottom rear bolt (1), 5-1/2” long (4413) grade 5 zinc.
  • Front bolts (2), 6-13/16“ long (16237-57) grade 5 zinc.