The limiting factor of the L98 is the intake. It will make a ton of torque at arelatively low RPM (really long runners), but falls off to the point of uselessness at anything over 5K (assuming 350cid). So with that in mind, you will be more or less waisting money on fancy short block parts and/or heads if you are going to keep the intake.
If he is set on keeping the intake, your best bet would be a mild cam, ported stock heads (they should be the alluminium L98 versions if the motor came in a Vette), nice headers, and maybe a better piston - but only if he wants to spray a little. Otherwise, I doubt you would ever make enough power to hurt the factory shortblock as long as it's tuned correctly.
TPIS sells a replacement intake for the L98 called a "Mini Ram". It looks very similar to an LT1 intake. That opens up a hole new set of options for you...but you will still be limited to tuning it with the stock computer which is the next biggest limiting factor to the L98!!
The quikest, cheapest, easiest way to power would be to smack a Performer RPM and a good 750 on there. That would be my advice.
I have a completely worked L98 setup and believe me, I've been down this road. If you need more specific advice give me a call.
Brandon 813 3680685