Actually.
http://www.ebay.com/itm/Qty-2-50UF-6...3D320606792940
At 10 bucks a cap, and the seller is in the US, I'll just purchase these. The seller has a 100% feedback rating and some vintage radio sites make mention of them on a positive light.
These high voltage filter caps are a massive pain in the neck to find.
I can't imagine subbing the 40uF with a 50uF will hurt anything on a filter cap like this. I subbed a 20uF 400V with a 100uF 450V once in a tube radio just to test it and see if it worked, and it did, and still does.