Hello Williamg........good to see you taking an interest in restoring some of the "olden goldies" :clap:
For what it is worth, I have restored/modified several of the older Monograms, and if there is anything in the following pics. that is of help,....... please use away !!.
All the early Monograms were somewhat flimsy at best and all really benefitted from "beefing up", not only in terms of handling, but, as you have discovered, in terms of strength,...as they did tend to Bend ;)
Two .055 rails have been soldered along the bottom of the chassis length.....this really helps not only with handling, but, strengthens some really weak parts of the chassis. On later versions I have run these rails to the front of the chassis plate.

A couple of .055 (.063 works just as well) "L" shaped bits of piano wire significantly beef up the rear uprights,.....and if you run the bottom rails (first pic.) all the way to the front of the chassis, the "U" shape bit on the front of the chassis is not needed.

A couple of "L's" strengthen the front uprights, and the downward legs of these "L's" can be trimmed to act as guide stops. (Your addition of a front axle tube does works equally well at strengthening the uprights)

The "newer" old Monograms had a removable motor mount, which was notoriously weak, and a piece of piano wire soldered here worked wonders........a piece of wire here on your chassis will help a lot !!

The biggest "Handling" tip on these old buggers was to cut/install a pan under the chassis to add weight/lower the cg.,......this one was cur from .063 brass, and mounts to the chassis with countersunk nuts/bolts.

A couple of shots of the finished rolling chassis.......the pan has been mounted with nuts/bolts, with urethane washers between the pan and the chassis plate to minimize vibrations.


Cheers
Chris Walker
Enjoy your build :dance:.....and post your progress !! ...............they can be made to run very very well !!!