Been there done that for a friends commercial van that now hauls a baby
Had old bench seat with safety belts attached to seat. Had friend weld
steel angles to seat base, it had originally been a hook type. seat a
little low so bolted it to floor adding 2 by 4 as spacer.
use several hardened boths per side with large washers on underside.
hardest part is deciding where to drill if the floor is flimsy use
metal plates under vehicle or even matching steel angle like used
drill angles clamped together so everything matches up.
in our case we moved the commercial vans security screen to behind the
new seat so stuff couldnt fly forward.
with the seat pre assembled with welded angle job took us about 8
paint stuff underneath to slow rust. do a nice safe strong job