My old 21st edition MH gives a figure of 0.00001244 linear expansion per unit length per degree F for aluminum.
Seemed worth mentioning since it offers a slightly different solution path.
www.roton.com is the website of a manufacturer of motion screws, and the site has a lot of handy and pertinent formulae for these calculations, as well as a beta version of a calculator program and a glossary of terms.
Job specific fixturing is possible for any rough casting. All you really need is the temporary services of a good process engineer to design a cost effective fixture, imo.
A bit late, but it might be worth looking at LaSalle ETD 150. The material properties are such that rifle barrels and actions can be machined from it with no heat treat.
you can get full thread engagement and a much stronger threaded joint by making your 5/16 section about 1/4 inch longer than needed and then using a piece of the 3/8 rod to make a threaded collar style spacer to epoxy in place.
in short engagement situations such as yours it's the absent first...
It's just an axially bored cone, center bore clearance fit on the bolt shank, with a single longitudinal slit to allow it to close down on the bolt shank as the bolt head presses it into it's matching taper seat in the mounting flange.
In effect, the tightened bolt becomes a defacto anchoring...
Where load must be shared between pins and bolts of limited numbers due to space constraints, the solution is often use of tapered bolt holes in the mounting and split taper seats under the bolt head to ensure that the bolts themselves have an initial significant load share.
Doesn't always...
Why not use a ball gimbal? They are almost by definition designed for axial loading, and are typically available in materials capable of high temp application.
A suitably large size self aligning pillowblock bearing would support your 250 kg handily, since you are not compensating for any gross...
