I guess it's time to put my JB Weld hat on a again.
I'm not going to repeat what I have posted before Pieter but if you care to look here
https://www.modelenginemaker.com/index.php?topic=7688.135 and scroll down to post 136 you will see the thoughts and practical tests I have done to prove it's suitability and strength.
My apologies in advance for posting pics on your thread but you may also find the following of interest to your build.
This was the first time I used it on cast iron both for cosmetic reasons - fillets etc and for bonding parts after previous use on model aircraft silencers.


And yes the engine is still in fine working order today but it has only run on air though.

This on the other hand is built from seven pieces of cast iron simply JB welded together the only screws (2 x 8BA) being to assist in holding whilst curing. This has run on 60 psi in a steam launch




You may not have seen it but all the pipework on my corliss engine was done with JBW as opposed to silver soldering and I have just begun another marine compound which again will have the major items made from composite parts JB Welded together. A slight mistake, smaller but identical in format to yours, has quickly been resolved with it too.
The comment about slumping is a relevant one - - there are two ways of dealing with it - leave the mix for an hour or so before applying it as fillets or filler (use it as mixed for bonding) or mixing some colloidal silicate in with it which turns it into a thixotropic mix that stays where you put it. Use the Standard not the fast cure and I've found it's best to leave it a good 2 days before
machining it.
Hope that helps with your build - Tug