I'm sorry but I just can't bring my self to advise the OP to use factory GM heads that were designed over 40yrs ago. There are so technologically outdated by todays standards it's not funny. Head technology has taken leaps and bounds over the past 20yrs. In my opinion pretty much any of the heads used in the 60's-70's are now obsolite. The only facotry head I would suggest to anyone now is the 906/062 vortecs, and even those should only be used when there is a serious budget issue.
The GM oval port heads had large runners but didn't have very good flow charactaristics. these issues acutally hinder making good hp/tq from begining to end of the power band. The whole idea of trying to buy heads is to buy the smallest runner heads that gives the best flow possible. typically if you can find a 290cc head that will flow nearly the same at .600" as a 325cc head then the 290cc head will make for a much broader power curve and make more usable tq since it creates more velocity at a lower lift and rpm.
take a look at these numbers:
all these gm oval port heads- 781,772,702,290, and even the 390's are all similar in there flow numbers. They range around 210-220cfm@ .300" and 255-275cfm@ .600". Now that is without any work being done to them, as they were cast.
yet you take a pair of aftermarket heads in the 270-300cc range and they are producing numbers in this area 220-235cfm@.300 and 325-350cfm@ .600"
they ones I would suggest are the Brodix 294's they flow out of the box 226cfm@ .300" and 337cfm@ .600". ****all of these flow numbers came from an independant site, not from the manufactures****
Then I can't bring my self to advise the OP to go with a cam that would be a "torquey" cam in a 396-427. The more cubic inches you have the more duration you need from the cam in order for it to react the same as a smaller cubic inch motor. A 210-215° @ .050 cam in a 396-427 will have a power band of around 1500-4500 but in a 489-496 it would have a power band of closer to 800-3800. Let me ask you guys this; if you wanted to build a mild "torquey" 383, would you use a 210-215° @ .050 cam that would be perfect for a mild "torquey" 327. No, you would use something with around 10-15° to achive the same kind of result (power band) since you have more cubic inchs. for you to have a power band in the 1500-4500 range, you will need to use a hyd roller cam with around 220-230° @ .050 in a 489-496.
I would estimate that if you used a set of GM oval port heads and a 210-215° cam that you will make around 380-400hp and 500-525tq. whereas if you use what I suggested you will be in the 475-500hp/ and 600-625tq range. simple as that, and it would still be very streetable.