There are many options for the 403 and most all the Olds families of motors. Unless, you have been asleep this last year, you would know about the Keith Black pistions, Bulldog Heads, Knowlton Head, Rocket Racing Heads. These are just a few of the new products. The windowed block is not a huge issue for a street motor. You can use a halo girle and mill the caps or run a full race girdle for extra insurance. You might contact FCR performance for you olds needs. The owner is very helpful.
www.fcrperformance.com
As for iron heads.......#4 is the best kept secret about the heads to use on the 403. Only problem is that they are scarce as hell. A one yr head on a 67 330 olds. Same motor the forged cranks are found.
olds