If I wanted to by a block to start a big block build, who would be the best manufacturer (Since dart doesn't make them) and where would I buy them?
Are aluminum big blocks available?
Are aluminum big blocks available?
The only thing I know of concerning that would be Ford had a habit of using to small a radiator with many of their applications back in the 60's & 70's. If you fail to change your present radiator to something larger than what you're running now with that 289, your plans for a big block or stroked Windsor will also include overheating problems. 
Over heating issues can be over come and shouldn't preclude someone from trying something different. 