chassis grounds on tweed twin kit.

Michael at Modulus Amps has developed and kindly drawn up an excellent wiring etc layout for a Marshall, it should be fairly simple to apply it to a 5F8A www.mediafire.com/file/arlsw02ata1j12m/Modulus+MR34+Layout.pdf/file
The download is safe, honest!

Personally I don't use the terminology 'grounding', because I think it's something of a misnomer, and leads to confusion and conflation between the amp circuit's 0V common and its mains safety ground chassis connection.

The 0V common of the layout above has a single connection to the chassis, at the input jack. That's a guaranteed way to get it right whilst also minimising the risk of RF etc noise entering the chassis.
The principal is to keep the HT current loops as short and contained as feasible, reference the grid and cathode 0V common returns of each stage to its HT filter cap's negative terminal.

Of course it's quite possible to build a perfectly quiet amp using other schemes, perhaps with multiple connections between the amp's 0V common and the its chassis, such that the chassis is used as a key conductor of its 0V system, that's how Fender did it, right up to the early 80s, even for amps with medium gain overdrive preamps, like my '75'. But if there's a 100 / 120Hz hum, such an arrangement makes it very difficult to track down and resolve.

For further reading, see Aiken and Merlin
www.aikenamps.com/index.php/grounding
www.valvewizard.co.uk/Grounding.pdf

Here's the way a modern amp should be wired for maximum safety.

AC mains hot wire through the fuse, to the switch, to the PT. AC mains neutral wire direct to the PT. AC mains ground wire to it's own ground directly to the chassis.



Some techs recommend soldering rather than affixing the ground wire to a threaded fastener because the fastener can work loose.  If you use jam nuts or Loctite it will not work loose.  Your call.

If there are two black wires coming from the primary side of the PT, there's no polarity. But you mentioned:

"We wired both black wires (the AC cord and black PT common) to the fuse, and both white wires (120V PT primary and and white from AC cord) to the power switch."

You should be connecting your AC power cord to only two wires (not two pairs) from the primary. Do you have a pic of the work? What PT are you using?

If you have other wires off the primary for other mains voltages, you need to insulate the ends and wrap them to keep them out of trouble.

This is the transformer in question. The primary wires are..

-Black Com
-Black/Blue (100V)
-White (120V)
-Black/Yellow (220V)
-Black/Red (240V)

So I'm only concerned with the white and solid black wire. The others are insulated and wrapped. I rewired it last night so that the hot (black) wire from the AC cord goes to the fuse, a jumper from the other side of the fuse to the first leg of the power switch, and the white (120V) wire from the PT to the 2nd leg of the power switch. The white neutral wire from the AC cord is spliced to the black COM wire of the PT and insulated with heat shrink. 

That's good work!

Everyone seems to do that. I think it's the worst possible choice but everyone thinks it right because everyone else does it.
The guys that design layouts are typically hacks without proper electronics training, but with good graphics skills. Regrettably, it seems to be the case for kit provider documentation too.
That's fine for pedal builds but not so much for mains powered gear.
PT mounting bolts will tend to loosen, due to the 3 forces of magnostriction, mechanical stress, and the lamination stack compressing over time.
Hence they may not maintain a proper chassis connection over time, even if Popeye has eaten extra spinach before he tightens them.
Your call, but for goodness sake, don't use a PT fastener for the chassis safety ground!
I suggest to connect everything to the bus, along the lines of the layout I linked to earlier. Michael is a highly competent electrical / electronic engineer.

Good call on not using a PT fastener for a ground. In a UL Listed Industrial Control panel, a ground can serve no other purpose.
Jim