What size you looking for? Aren't there already BBC marine inconel's available? If you are into repurposing, diesel and industrial applications use them if you can find suitable dimensions. If looking for expendable throwaway ones, I've seen them nearly free from major rebuilders by the barrel full. Contact any inconel valve seller to see what their current offerings are. [A brief flip through a catalog shows 7 pn's for common BBC apps.]
peaked my interest about Inconel. looking at the Inconel used, it certainly not like the Inconel we used at pratt whitney. one site said machine like normal stuff. can grind or weld. jet engine Inconel definitely is not like it. jet engine inconel can't be welded or ground. it will produce 'white layer'. 'white layer' produces cracks that eventually gets bigger cracks, blows up and plane crashes. jet engine Inconel is 4-6% machinability. we use to make inserts out it to show the generals, etc how tough this stuff is.
I wouldn't think any of the manufacturers have the correct length 1.75 valve to fit a BBB. Especially not from a BBC with the 1.75 size
A quick google shows at least 12 grades of Inconel. Different grades required different approaches in the various parts 'we' made. http://www.californiametal.com/inconel.htm
I know that Manley has stuff for at least BBC and SBC, I bet they have more. Mopar 440 has similar size and length as Buick. The reason I pointed you towards industrial/diesel is that with the massive quantity in use they aren't always as expensive as the $50 ea. Manley's, and may be close enough to modify. They are indeed grindable, they just take a bit longer and you don't get more than one valve out of a stone dress. They do load up the stone somewhat. (referring to a very light regrind of a million mile valve) The high end valve makers can probably hook you up, such as Del West or Xceldyne.
If you are looking for the best, call Victory. Not sure why you want inconel valves unless you are running nitro methane, a turbo charger with lots of boost. There are many reasons to run something other than stainless or titanium. It is best to talk to them about your engine combination and use. I bought inconel for Johns nitro motor, which was put on hold since we could only get high 440's cfm from S2 solid TE heads. With our new billet heads that flow in the 530's right out of the box at high lift we can now compete in nostalgia top fuel. We will start with methanol.
Just curious, I'll have the capability to lay 1700 wheel HP on high boost, but I probably won't ever push it that far, I'm going for 1k to 1200 at the ground with pump gas and meth I know it shouldn't get too hot with that setup, but if I did want to turn it up to 25+ psi and 2k+ HP I don't want to loose sleep over the valves
Installing a set in customers heads as I write. Just had them made for this job. Took about 2 weeks to get.