he had managed to undo the bolts, which you would appear to have done by removing the heads.
To 'break' the joint between the gearbox flange and the joint, this is what he did
Try using a heavy copper hammer, and wear safety glasses
If that doesn't work drop the van back on its wheels and select first gear and 'drop the clutch'
then repeat the action in reverse. You will not sheer the bolts by doing this as that is there strength
but the rapid force normally breaks the adhesion of the mated surfaces.
jack it back up and secure it.
Inspect to see if the two parts are separated with the hammer,if not repeat the process.
the note (lack of ring) should tell they are no longer joined.
If so, work the faces apart so as to give enough space to pass with the angle grinder
Then give the angle grinder to the bloke who seems to know how to cut things
Remember to give yourself enough length on the threaded section to be able to get on them
with the vice grips, wont be tight. You'll know when the last bolts cut through
Of course I am not promoting the use of such methods, as some may forget what they were doing and
end up at the shops.
good luck