There's a guy with a twin-turbo'ed Cummins that can get up to 100 psi of boost.
For a 617 (or just about any IDI engine), the big limiting factor for boost is the compression ratio. More than 15 or so lbs of boost will just put too much strain on the head gasket and on the head bolts.
So, the solution? Lower the compression ratio
I don't think that putting a thicker head gasket would be that great of an idea, simply 'cause it probably wouldn't have the needed strength to hold the added boost. So, the best way would probably be to slightly shorten the connecting rods. There's a guy who's gotten a 6.9l IH IDI diesel (like the one in my truck) down to about 17:1 compression, and unless it's a really cold day, he sayd it'll still start on the glow plugs without needing ether.
So, get the compression ratio down, and put head studs and a heavy-duty head gasket in, and the sky should be the limit as far as boost goes.
Then all you have to do is make sure that the intake and exhaust have as few restrictions as possible and add enough fuel to make use of the extra air (through bigger injectors, ALDA adjustment, etc).
Lastly, make sure you have an EGT gauge
Yes, I've got ideas for when I have $$...
