Ok, so I'm back on using a stock block. I just see threads of split blocks even at relatively low power numbers and get scared. I broke the crank and block in my '85 GT and it only put down 220hp and 270ftlbs to the wheels. I was just driving it with the cruise control on when it went.
There is a very good reason for that. Nobody starts a thread for a block that didn't split. I put almost 500hp to the tire on a 347 stock block, and it lasted 85,000 miles before it had a failure unrelated to the block. I didn't make a thread for it.
300 whp on a stock block is well within the safe margin. With TF170cc heads, I would stick with the stock stroke. Those heads don't flow enough to make a 331 worth it. I would use the money towards a good tune. Don't rev it up too high. High RPM is usually what splits a stock block. With TF170 heads, there won't be any reason to rev it higher than 6500rpms, not even that high really.
Kurt