Last summer I installed jump seats into my 86 Syncro.  The following tips
should be helpful.

When you do the jump seat install, first place ALL components together in
the van for rough location measurements.   Next, drill holes and bolt the
bracket to the battery box, being sure to use a flat steel plate inside the
battery box for reinforcement.  Then temporarily insert the seat so that the
spring loaded pin is secure in the catch of the battery box bracket.  Next,
place the interior floor brackets (with protruding pipes) on the floor with
the seat in its intended position (cushion flipped up is easiest for
measuring) with the leg pegs inserted into the unfastened floor brackets.
This allows accurate measuring of the floor bracket location, since it is
not quite as critical a fit as is the spring pin/battery box bracket.

If you permanently mount the floor brackets first, you run a distinct risk
of being ever so slightly off in the measurement for the battery box
bracket, which results in the spring pin not fitting easily into the battery
box bracket. (BTDT!)

This technique should be followed regardless of the type of mounting (bolts
or welding) used on the floor brackets.

For those of you (like me) who obtained a used set of belts minus the floor
brackets, don't despair.  You can easily fashion them by using flat steel of
appropriate size, unto which you (or a shop) have welded a short length
(3/4" or 2 cm) of steel piping that will fit around the pegs on the seat
legs.  Since the floor of our vans is not flat, be sure your steel plates
are properly shaped, as a simple rectangle will not quite fit.

A note about seat belts.  It is not necessary to use the OE belts.  You can
use any standard lapbelt, as it bolts into the existing hardware.  The OE
belts mount exactly the same as a belt available from your FLAPS.  The belt
for the outer side of each jump seat bolts into the outer L shaped (*) part
of the battery box bracket.  The inner belt for each side shares its
mounting bolt with the existing front seat inner belts.  The only advantage
of the OE belts is that the outer belt has an automatic reel for tension and
the inner belt is very stiff so that it doesn't get in the way of the center
passageway.  These are nice benefits, but hardly worth the dealer price for
me.

(*) Note:  If you don't have the L shaped part, it can be easily fabricated
out of a flat steel piece, by giving it a right angle bend and drilling
holes for the bolts that secure it to the outer end of the battery box
bracket, and a hole for the seat belt bolt. Remember to always use
appropriate size and strength bolts for all of these connections.

Since these seats are mounted backwards, the forces of a frontal collision
push the passenger into the seatback.  Thus the need for a shoulder harness
is reduced, IMHO.  Although I understand the desire to safeguard any
precious cargo,  a discussion of shoulder belts is not covered here.

This install can easily be done in a few hours by anyone who would even
consider the attempt.  Careful measuring and a few tools are all that is
required.  It is a wonderful convenience to be able to remove/install the
seats in a matter of seconds without using any tools.  These jump seats
really are a pleasure, especially if you have passengers who don't mind
riding backwards.

Budd Premack
86 Syncro, 73 Super
Minneapolis, MN (Land of Sky-Blue Waters)