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....we never use welded joints anywhere in my job (huge majority of it being ally machinings) so my experience designing structures for them is low - but, you should be ok...it's always soo hard to know with cars what loads they are going to take when you don't work for the OEM! 
then weld on the relevant bracketry if required to mount to the engine and what not. 
Might be worth getting this fabricated at a machine shop as I'd also suggest a full penetration bevel weld each end of the tube. If you can supply the dimensions and angles then I can knock you up a drawing if that helps.- I did think about this last night but decided against suggesting it because; You've already made a cradle! You may have trouble mounting the engine to the tube - it's easier with the square section. Likewise, you may have trouble getting a good joint on any seal plates you might want to weld onto the cradle. If you are not careful you could reduce the tube materials strength properties when you bend it into a curve - I would want to be doing the bending on a proper machine, or when the tube is very (very!) hot so as not to pre-stress the part. Your faceted shape could be more space-efficient than a curve.- so if you think you can overcome 2-5 and don't care about 1 then go for it! You might want to do both solutions to see which works best.... i have some thick walled 2" tube i could heat up and bend to shape without too much trouble, and weld some platforms on for the engine mounts, that is not a problem... decisions decisions! my local roll cage fabby does bends at about £10 per bend on a CNC mandrel bender. even the most expensive places are only around £20. youd only really need 2 fairly shallow angled bends. the whole bit of tube with bends would cost you under £40 potentially if you find somewhere reasonable. fairly steep compared to your current method, but it will be much stiffer, easily strong enough that it should never break, and will look proper sexy in the under-car shots of the build later on 
...i am just a bit busy with customer conversions to give it any real attention at the moment.My VEMS ecu and converted Audi engine harness is due back in the next fortnight, so that will be ready to plug and play.I have my modified fuel tank to refit soon and when that happens i should be within a matter of days before firing it up for the first time, although the gearbox needs to come back off to fit the flywheel and clutch assembly.... over the next month i am going to step up the amount of time the car gets so that i can make some real progress...(new T off position)(Early prototype is here but needs redoing http://www.youtube.com/watch?v=Q3B25W3Yeds)(currently no solution) 
/ trackday. I suppose it is also a good promotional car showing what we can do too.
| 99 528i SE Touring | 01 Mandarin VX220