Craigslist 4 Speed - Something doesn't jive?

All the Mopar boxes for GMs that I have seen were overdrives.
To use the 3.09, you have to take all the guts. The 1x23 input from a Mopar box may or may not sub onto that cluster.I do not recall the tooth count in the GM box. I suspect it would. I also do not recall if the Mopar retainer will bolt in place of the GM.
IIRC no other input gear, that is to say; only an input gear from a 3.09o/d box will fit on that o/d cluster, and only if it has the same tooth count(there are at least two).
So you might be stuck with box that has a too-short 10 spl input.
I guess you could buy an adapter bell and mount that GM onto the back of your SBM, but it would still have the long tail.This will not allow the use of the factory floor hole, without some custom fabrication, if indeed it can be done. Most want the stick to drop down in the factory hole.
The Mopar tail and output shaft can be swapped onto the GM box,giving you the proper shifter pad.But not just any short m/s; there are 3, and only a m/s from an o/d fits into a m/s third gear, the o/d gear. There are two; a 727 sized one and a 904 sized one.
I have not seen a 3.09/direct 4th for a GM application. But that gearset is a set unto itself. Only the m/s low will sub into any other Mopar non-hemi,box.
That is going from memory, back to the late 70s when I was an employee in a rebuild shop. The m/s low is an across-the-board swap.
If you don't already know; there are 4 Mopar gear-sets with Direct 4th output and two overdrives.
1) the Hemi stuff which will not interchange with anything else.
2) most common wide-ratio set with a 2.66 low
3) the 1970 T/A close-ratio set with a 2.47 low
4) the early-A,273 deep-low set with a 3.09 low
5) the common A/F .73 overdrive with 3.09 low
6)........a swapmeet .71 overdrive with 3.09low;I ran across and purchased for parts to swap into my blowed-up .73 box and nothing fit!
7) the GM box AFAIK is the same as #5

There may be more, these are just the ones I have actually laid my greasy hands on. I have owned at least one of every example, except the hemi-box.

The 3.09/Direct 4th is IMO a great set for a streeter with a cam up to about 250@050. It allows you to run a decent rear in the range of 3.23 to 3.73. The 1-2 split is 62% so just about anything will pull that. And second gear (1.91) is good to 65 or more mph with any of those ratios.
3.91s makes the starter-gear a bit deep at 12.08, but it does max out the Rs nicely at the top of second,at about 6000, for a 230* cam; so a traction aider will minimize your zero to 60 times.Otherwise expect monster tire-spin.