My 2 cents would be....
1. Any production based small block ( 273,318,340,360 )
2. Any non offset rocker aftermarket or oem "street" head... iron or aluminum ( Eddy, Speed Master, Promax, EQ ect )
3. 600 lift max on cam... rollers allowed to let the magnum block in
4. Any RPM range...if you have the stones to spin it have at it
5. Max horse power/torque per cubic inch calculation FTW