Cable ties work fine here. It's only vacuum. But only use black ones, the white ones degrade with heat and light.
If it was good enough for OMC to use em for fuel lines on their 1970s -80s Johnson & Evinrude outboards, I have no problem.
They never leaked fuel in the 30 years I had boats.

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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.

Wasn't the breather on top of the flywheel/clutch housing put there to relieve pressure on the primary gear seal? If you block it off does this increase the pressure?

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Slide
1965 Mini Van (1360)
1970 Mk2 Cooper S (1310)
1978 Rover SD1 V8 (4.6)
1996 Land Rover Discovery V8 (4.0)

No it was put there because 1100S etc motors have no sideplates.
Crankcase pressure there is same as elsewhere in the sump.
It's not a great spot for a breather, because oil gets chucked up it at high rpm on LH bends.
Like at the dogleg at Wakefield Pk, my car made like JB007 for 50M after.

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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.