IMO;Not too good; the ratios are too far apart. Unless your 360 was built for torque
The ratios are 3.09-1.67-1.00-.73od. And splits of .54-.60-.73
Lets talk about these splits for a sec. If you take the Rs out to 4000 in each of the first two gears, then the Rs will drop to 2160, and 2400, so your 360 will need some torque down there, to accelerate briskly.
With the 2.66low box having ratios of
2.66-1.92-1.40-1.00, and splits of .72-.73-.71 , the Rs will drop from 4000 to 2880 and 2920. These are fully 33% greater and 21.7%. So just like a hi-stall TC allows your engine to spool up to where there is more power, so also does this 2.66 box allow your engine to remain at a higher power level. So if you already have the 2.66 box, then a straight swap to the od box, while it will launch better, the advantage is lost right at the 1-2 shift. To get any advantage from this box, you need to also change the rear gear.
So lets look at that.
Assuming:
Those tires are 26.7 tall and have a roll-out of ~84", and with 3.23s ,and the od box,and say a torque peak at 3800, and a shift rpm of 4500,
then your speeds at the shifts are 36,66,111,and 152lol. Basically you're asking the engine to pull the load to 66mph with just two gears;then skip direct,and when you drop it into Overdrive the Rs will fall to 1956@66mph. Has your engine got enough torque at that rpm to continue pulling? I venture no. At least not at a small enough throttle opening to be fuel-efficient. So what's the answer?
Well, it might be more gears, so you can use three for acceleration and one for cruising, instead of skipping one.
If you found that your torque monster 360 could get pretty good fuel mileage at 66=2600, then 4.30s would get you that. Shifting at say 4000 now, because you have so much more TM, your shifts would occur at 24,44,74, and 66=3570 in direct, then 2600 in od.Now you are using 2.5 gears to get to 66. With a torque peak still at 3800, you have some passing power and hill-climbing ability in direct. I chose 4.30s because the tooth count is 43/10 and so it is a good strong nearly-impossible-for-you-to-break, ratio... 4.56s would be better but the tooth count is 41/9 and so a bit weaker ratio.
The other option, if that trailer is particularly heavy, or the 360 is weak, would be an early 4 speed; 3.09-1.92-1.40-1.00; and the 3.23s (that you already have). With this "close ratio", now, maybe you can shift at 3800,and your shift speeds would be 30-49-67 and then into direct to cruise at 66=2700. This would be using three gears to pull to 66. Of course passing would be 66=3800 in third gear, bang on the torque peak. The starter gear with this combo is 9.98 so it takes off like a 2.66 box with 3.75s. So it might take a bit of clutch to get moving. 3.55s would be 10% better. I know that doesn't sound like much, but if you had 200ftlbs at 2000rpm then 20 more might allow slipping, from down a couple of hundred rpm. Which means you don't have to rev it up and sound like a Mustang.
With the od box/4.30s,I would run a very small 4bbl. cuz the starter is 13.29, so you will have gobs of take-off power,and the only time you will need a 4bbl is for passing, and you'll be doing about 3200 to 3800,in direct from 60 to 70 mph. Mathematically this requires 417 cfm@100%VE. So a 500 is more than you need
Here is a comparison of some ratios using total TM.
First; the 4.30s and od-box ; 13.29-7.18-4.30-3.14
Next the 3.23s and early box ; 9.98-6.20-4.52-3.23
Next; 3.91s and the 3.09box; 12.08-7.51-5.47-3.91
aaaaand 3.91s with 2.66 box; 10.40-7.51-5.47-3.91
aand one more the 3.09 low; 10.97-6.82-4.97-3.55
As you can see,in the first two; third and fourth are virtually the same, while the od box has a granny low gear, and a 15.8% better second gear.
If fuel mileage matters to you, then I would chose the early A box because you will be hauling with the power going straight through the box in direct fourth. The extra 100 rpm at 66mph shouldn't count for much.
If you need more power to pull at 66mph,then more TM could be the answer.
But if you also need more power at take off,to pull that 9.98 first gear,then I suggest more cylinder pressure.
With the od box you are already maxed out on TM on account of that granny gear.
Whereas with the non-od boxs you still have about 20% left to go . This would get you 3.91s and so; 12.08-7.51-5.47-3.91 with a cruise rpm of 66=3240. (28" tires would drop that to 3060). Passing gear would be 4540@66mph, probably a lotta noise with not much go,using the stock 360
I think the 3.55s would be about the best;10.97-6.82-4.97-3.55; passing gear of 66=4120, a starter of 10.97, and shift speeds of 27,43,59 @3700, then into 4th, or just hold third to 66@4120, then into 4th for 66=2945. 28s would drop that to 2810 With this combo I think I would use a small spread-bore carb with really tiny triple-booster primaries.
Hope that's helpful