how much will a 69 block handle with forged internals?

I'd put a girdle on that block man. VERY important. The 69' Block is a great block and should handle 650-700HP with a girdle on it. I run a 69' Block and I use a Probe Main Girdle which fit great. I then purchased a Universal Windage Tray Screen from Moroso and made my own windage tray to fit with the girdle. This required using 5 3/4" long ARP Main Cap Studs. It was a pretty nice setup when I was done.
You can also get a nice Girdle/Windage Tray setup from D.S.S or Ford Racing if you don't like fabricating.
Overall though man, I don't care what anyone say's... with that kind of power you have got to run a girdle to keep your crank and block in one piece. 650HP shoudn't be a problem for the block.

Throw a 150 shot of spray on her.