Bertram 42 Detroit 6v-92TA Repower with Yanmar or Cummins

What is the current gear ratio on the DD 6V-92?
It is a 2300 rpm and you should try and match the propeller rpm with the new engine in order to use the same running gear. You also want too make sure to size the new shafts correctly for Diesel engines, with the right shaft material and safety factor of at least 5.0.
Propellers get a bit trickier with different engine power curves, but you should be ball park on diameter by keeping the prop roms the same, for the relatively same bhp of the three, and some minor adjustments on prop pitch and/or cup.

Can't comment on Cummings engines but we repowered a Huckins with yammer 480 hp and they have been flawless aince new .just did the 1000'hr service and they were perfect . We did also change shaft,p bracket ,props etc .

2300/2.0 = 1150 shaft rpm.
For the Cummins, 2600/1150 = 2.26 gear ratio.
So continue with a gear as close to 2.26 as possible. ZF will be lighter. Not sure if you have enough room for a flexible coupling and you certainly do not need it, but make sure to have the shafts checked for straightness and the ends checked for run out during the process.

I am surprised that the 6V-92 has a gear ratio of 2.0/1, would have expected more like 1.75 or even 1.5.

I would double check just to be on the safe side.

Do you have a picture of the Allison gearbox name plate - it should have the gear ratio stamped in place?

Every Allison M20 I've ever seen is 2:1 ...M15 1.5:1

I do not think it is a down-angle gear but there is a horizontal offset, as the input shaft centerline from the diesel end is higher than the output side of the gearbox flange centerline.

It is best to make a jig or mock-up of the original versus the new engine to understand if there will be any engine stringer or engine mount modifications necessary, and there usually is.

Yes, the assumption is that the OP will be replacing his 475hp DD 6V-92TA and Allison M20 gearboxes with a 493hp Cummins QSC 8.3 and either a new Twin Disc MG-5065A or possibly new ZF gearbox.

Are you looking for extra room as in height? As stated, the twin disc gear will be a "drop down" or vertical offset from engine crank centerline to output flange centerline.
The down angle you are talking about may allow for a more level engine installation measure for an aft if you need it. There are technical requirements for all these parameters, and a lot is dependent on stringer height, shaft angle and the centerline of the shaft coupling distance as measured from the interior hull bottom. That is why I like a mock-up before final selection of all your gear, especially to understand the new relationship of your Cummins engine mount brackets, gearbox mounting brackets and their resilient mounts. You have some geometry homework