Ive been reading and its all uk stuff and not sure if it the same in aus.

If i get a 1275 it "will" bolt straight to my old gearbox but i need drop gears for a 1275 correct? what about the clutch?

Is the 1098 a Mini 1100 motor, or is it out of a Morris 1100?
Either way the 1275 will bolt straight on and the crank will turn, unless the gearbox is an early Morris 1100 3 synchro.
You will need a 1275 primary gear, but the idler gear and bottom drop gear are fine.

But if it is an old 3 synchro one I would bin it and fit a 4 synchro remote box, or a rodchange one. Both are stronger and easier to get parts for.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

gearbox is from a 71 roundnose, no idea if its standard. its a remote box.

Is it worth me getting a rod change box?

OK it will bolt straight onto that. It's a 4 synchro remote. I put my 1412 stroker onto one recently with no case mods.

These boxes have the same gears inside (except reverse) as an Oz rodchange box but they are better located (extra mount at back of remote), so unlike a rodchange you don't need more than the std steady to stop it rocking about.

Better than a rodchange box, IMO.[/flame suit on]

[edit] I would clean the box out, check & replace worn bits before sticking a new 1275 on it...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Excellent, Thank you sir.

Saved me some money on a gearbox

You would want to put a 3.44:1 diff in it, that 1098 one is a 3.647.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.