That's far too tight for that little nut. We are talking Camshafts?
A bit late now but contrary to the official assembly instructions, Service Sheet 208, the camshaft and its drive gear can be assembled into the timing side case before the cases are joined.
Make sure the locating key hasn't moved back to gouge the face of the bush, and that the camshaft gear front face is above the end of the plain part of the shaft. Other way round and the gear isn't going to get tightened against the cam and the drive will be through the relatively weak key alone.
The cam can be easily held still with an adjustable spanner, padded if you are fastidious with a piece of card or similar. Then tightening the nut is easy.
With the cam already in place, the same trick is preferable to blocking a lobe against the crankcase, as your enthusiasm is more than a match for somewhat weaker 60 year old alloy. Finding a suitable spanner is the hard part. It needs to restrain the cam but put no load on the case to avoid damage.
With no official torque setting, Loctite on the threads, plus the tab washer, then a good firm nip up usual for that size of nut should be fine.
I see this is your first post. Head on over to the intro section and tell us how you got into this mess.
Welcome along
Swarfy.