should the head studs for a alum. block have a longer thread engagement than for an iron block?

Ive asked the same thing before and was told to use the standard studs for whichever heads being used, i would think to use all the threads would be best buth that would require custom studs and remachineing of the deck

You should use the studs designed for the block and yes they might have much more engagement.

The KB aluminum blocks are drilled and tapped for longer stud engagement in the aluminum. It is about 1.6" vs 1.0"
The KB studs (made by ARP) have a coarse end slightly over 1.7" long, but it has a non-threaded end/tip about .1" long.

I don't think the Indy blocks allow for longer stud engagement.

I have a world block and the threads go a little over a inch longer than the bolts.

KB is a different animal and requires it's special studs. If it's the World (I have one too) use the studs made for the heads.

If the mfg recommends a specific stud then I would use it. If they don't I'd measure the bolt hole and use a stud which fit the head and had at least 1.5D worth of thread engagement into the block. 2D of thread engagement is probably better but you need at least 1.5D. So for a 7/16 bolt you want at least 3/4 of an inch of engagement and 1 inch would be good.

Another engineering rule of thumb is that you need 8 full threads of engagement to transmit 100% of the torque. On a 7/16-14 stud that is 8/14 or 0.570 inches of stud engagement. But you need to add a safety factor to that since there is usually a chamfer on the top thread.

Yea, they are really built. The main caps have studs that are 5/8" in the aluminum block, and tapered to 9/16" through the caps. Way bigger than the factory 1/2" main studs.