Yes a 1275 will bolt onto a 1098 box.
Here in Oz we got the Morris 1100 3 synchro, and Morris 1100S (1275) 3 synchro (not 4 synchro A/A box, but same motor). Both used the same gearcase. I have one box off each under my house. Identical.
[edit] p/no 22G68.

I am not sure if 1300s bolt on to 1098 gear boxes. 1098s have a longer stroke, but 1300s have the big big ends and the big connecting rods. You should really just offset grind the crank to regular mini or Cooper S rod size and use those rods and have 1098 ish stroke and all the extra cubic centimetres etc. OK, I have done enough here.

Please dont curse me with an appearance from the velcro and WD40 idiot!

The AA automatic block will bolt straight up to the AA manual gaearbox case  - but not the skinny 1100 original case without a bit of grinding which may weaken the case?

The conversion of autoblock to a workable manual block is fairly exacting but it has been well documented - for those who are interested this is a very good website.

//home.earthlink.net/~roygmisc1/convert1/convert1.html

One just has to be fairly exact on the measurements of the plate and of course drilling the hole requires one to get your tongue sticking out at just the right angle while drilling

cheers

Barri

Todd Miller's Austin America website details the steps to mod a 1275 auto block for manual. It involves drilling a couple of oilway holes and making an oil pump spacer up. I provided the pics.
I've been running one in my Mini for years.

//www.austinamericausa.com/ 
[edit] scroll down to technical advice, engine.

beyond the ability of a fool.

My curious side says to ask for an explanation......

The automatic engines bolt right on to the manual transmissions. Sadly, they do not automatically reconfigure themselves to get oil to flow from an oil pump, through a filter and into the engine. There are parts to buy, rituals to be followed, gurus to be consulted. It can be done, but it is a job that rates a six on the one to five Veloco Robster scale.

Others are better qualified than I, but I don't think you can put the 1275 from an automatic on a manual gearbox.

Thanks very much guys - appreciate the advice

I will go with the AA box under the 1100 and put the 3.44:1 FD in the car. If I can find a lower FD ratio I will look at that right down to 3.1. I dont want to be stirring the stick too much on my twisty home roads but probably 70% of the driving will be at 60mph + freeway speeds.

If I get it done in time I also have a 1300 (1275) AA automatic engine that I will convert to manual but for now will probably only have time to use my 1100 if I want to be driving by summer

thanks again

Barri

A 2.9 is a good compromise Mike. The 2.76 works fine in normal situations but i found it requires more down shifting in the steep hill country. With an 1098 engine i would at least try to get to a 3.1 gear.

Ever since I drove a 1275 with 2.76 final drive I have been sold. I am doing a transmission right now. It is difficut to fine a 2.76 gear et so I am using a 2.95 set. There is plenty ofacceleration but I am too old to bea street racer and it is fine for me.

I've always used 16mph/1000rpm for a 3.44 final drive with 10" wheels.

The man clearly wrote that he has an 1100 drivetrain with the very low 4.13 final drive. He has an Austin America transmission, and correctly states that it has a less low but still unpleasant in a mini 3.65. He now has a 3.44 in hand that he could also install. He would like to make the car more driveable.

The 3.44 final drive will do that.

He could try and find a Jack Knight input gear that changes the drop gear ratio, or he could find a 3.2 3.1 2.95 or 2.76 final drive. The tall final drives require a slight opening up of the third motion shaft bearing retainer, which is frequently discussed.

The long stroke of the 1098 makes for a torquey engine, and having too tall a final drive under my own car I would suggest a 3.1 or a 3.2 final drive, or that fancy gear and the 3.44. I also would suggest the Austin America 4 synchro gearbox as I think the 1100 items are three synchro and have the small idler gear. 

Knowing the factory at time put a 2:76 under a 998  ... and the car still got around at low revs ( slowly )

- I would go with the Lowest FD you have on hand, plain and simple.

you're used to it Zinging -- drop it down as much as possible , Even a 3:10 if you wanna go lower. 1100 isnt an acceleration monster anyhow - and should have good torque to still pull a lower FD...

Are you sure your speedo is correct?  I'm faily certain that a MINI on 10" with a 3.44 FD is pulling 4000rpm At 100kph ( 62mph)

Remember the 1100 and other ADO16 cars used 13" wheels.....so with a lower FD they are still pulling MINI revs.

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I am trying to make my 1100 into a more forgiving occasional daily comuter. Currently I run a standard 1100 engine and transmission and the car is great but the 4000rpm at 60 is killing me. I have a second 1100 engine and transmission I am busy refreshing (new bearings, ring, valve job etc) which I will drop in once done. I was going with the original transmssion and i have a 1:3.44 finl drive to replace the current 1:4.133 FD making for about 3600 rpm at 65.

I was going through my "pile of stuff" and pulled an austin america transmission which has a 1:3.65 FD and slightly different gear ratio's

Gives me the option of going with multiple combinations of original 1100 box with either 1:3.44 or 1:3.65 FD or put the AA box under the 1100 and run it as is or with the 1:3.44 FD.

The gear ratios are attached straight from the book - any thought on drivability greatly appreciated.

cheers

Barri