I probably can't help with the actual casting, as my current equipment is setup for much smaller, higher tolerance parts, but I can give some advice.
The idea of spincasting may work well indeed, at least as far as solid investment system processes go. Vacuum or similar wouldn't cut it with pieces that large and with such a cross-sectional density.
I would however, not go there as a first option. I would look into a zirconium oxide shell casting system as the best suited for this type of work by far. Any art/sculpture foundry will use this method for bronze and stainless casting.
See if there are any foundries in your area, or if all else fails there are a number concentrated in a place called Loveland, Colorado which seems to be sort of the capital for art casting in the U.S.
They don't advertise for smaller jobs, they usually don't want them, but they are very capable of doing small pieces very cheaply - as long as they know you are willing to wait.
Otherwise, the -minimum- pour size they do is something like 600+ pounds usually. Unless you want to pay the charge for a scheduled pour, little things like car parts tend to sit on the shelf until they can add them into another batch of pours. Could be a day, could be a month, but they are worth the wait.