Two summers ago, I built a 350 for my 88 Sport Coupe. Three weeks after I put that motor in, some lady hit me and totalled the car. I now have a 1988 IROC-Z, 5.7 TPI. [L98]

I want to keep the motor TPI; actually, I have to for emissions purposes.
Unfortunately, the motor I built was carbed. Here are the specs.

350, mid 80's 4-bolt main
10:1 compression
Vortec heads
High-rise Air Gap intake (1500-6500 rpm)
.488/.510 cam (1500-6500 rpm)
I don't think it's a roller cam.
I had headers, (w/o AIR tubes), 3" exhaust, Flowmaster 80 muffler.

This set-up was probably getting 350hp and 400ft/lbs torque. It took off whenever I needed it to, even cruising 70mph on the highway.

Now, I was planning on taking the motor I built and just swapping out the top end of the L98; simply bolt on intake and heads. However, the L98 iron heads aren't as good as my Vortec heads, and I'm worried that the powerband isn't set up the same so I won't be getting it's full potential.

Another thing I realized is that I can now get a higher lift roller cam because I now have roller lifters in the IROC motor. Just throwing that out there as well.

In the end, I would love this setup to have a good amount of torque in the low and mid-range RPM. I want it to take off from a stop, but not completely die off if I'm cruising down the highway. I want to keep as potent, if not a little more, as the motor I originally built. This car will simply be a fun cruiser, tire spinner, and possibly show car. I don't really plan on racing it, track or street.

Any help is appreciated. I really don't want to spend a ton of money.

Thanks again. -Matt

edit:: (2.10.12)
Upon doing a lot more research, I realize what I actually want. I want a good amount of torque on the low-end. I won't really get over 4000 rpm anyway, so maximizing the low-end is better for me than pushing my powerband higher.
I am most-likely going to keep my stock intake so I can keep the powerband on the lower side. However, am I restricting it drastically by not changing anything? I will probably also keep the stock L98 iron heads. There is no point in me machining them, considering that will probably cost me the price of a set of new aluminum heads.

So I have some questions now.
If I keep the whole top-end stock, but change out the bottom end for 10:1 compression and a bigger cam, should I expect a decent gain in torque and hp? Especially in the lower-end?
Is there anything worth doing to get a boost in power? I don't want to spend $700 to just get a barely noticeable gain.
Will it be worth it to take advantage of my roller lifter set by buying a new cam for it?

Also, my mechanic said all I have to do to get some gains is put on a throttle body spacer and a bigger throttle body. He said he definitely noticed a difference when he used to build these motors when they were new in the 80's. Is this true at all?

Sorry for all the questions, guys. :/
I'd normally ask my grandfather some of these questions, but things are a little hectic right now.

Thanks again in advance.