Is there any difference between a small block box and a large block box

theres a few different types, rod change, remote change, pudding stirrer

i am pretty sure that any engine can bolt on to any type of gearbox

cooper and cooper S and GT has different gear ratios

there are different diff ratios too

Not quite right. But you're close..

If talking 4 synchro boxes- whether remote or rodchange, any engine including 1275S or Morris 1100S will bolt on and the crank & rods miss everything.
These days, most people are using these boxes, so no problem.

But, with 3 synchro ones it's not quite that simple- the old 850 boxes foul the 1275's conrods big time- not worth trying. Some of the other cases need a bit of grinding here and there for things to miss..

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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.

thanks doc, try to remember that