Is the gearbox tag in good enough condition to examine it and read it?.
I would hate to suggest the following but if you cannot find left ahnd threaded bolts for the unit you may be better off buying new arbors and bearing hubs for the mower as they are easy to obtain from many places.
You may just luck out and find everything you need at
Agri Supply - Farm Supplies, Tools, Lawn Mower Blades, Cast Iron Cookware
1800-345-0169 they have lots of mower spares
saying that and making sure I did not brush my teeth with gun powder this morning-
1. as you know the threads are left handed;
a.be sure to wash out all the holes with WD-40 or alcohol to clean them.
b. use a new unsharpened pencil to determine the depth of the bolt hole-
dont hesistate to scrape around the bottom to dislodge any rust or cob
webs.
be sure to mark the depth on the pencil in the blind hole with a pencil or pen or scratch awl. where the pencil clears the arbor.
If it were me and was limited on tools and such I would find a machine shop bribe with ample donuts and coffee to ask for a minutes help as you are in a bind (if Agris upply cannot help you-but i would ring them up first.
Please understand that if you require a "compression washer" spring to
hold the bolt in place you will need to add length to the measurement of
the bolt and each time the blades come off you may need new washers-again agrisupply can probably be of help
these bolts are left hand thread to counter the G forces of the rotation
and in the process keeping the blades tight
c. are you sure you do not require a "compression washer" to gain more
pressure against the blade as is done with some mowers?