With the stock gears (3.08?) and the TH-350 you will be lacking in low RPM power and torque to achieve 425-450hp.
If you go roller you get more power under the curve, better vacuum and can reduce the duration to get the same power.
Maybe something like this;
http://www.compcams.com/Company/CC/c...?csid=160&sb=2
Still lots of overlap at 65* but with roller you will get more vacuum than with the flat tappet equivelent.
Or maybe step it down a bit to this cam;
http://www.compcams.com/Company/CC/c...?csid=159&sb=2
Still good duration but now you get more low RPM torque and less overlap at 59*. Not sure if it would get you 450hp, kind of depends on the heads. They make a big difference in your ultimate power production and power through out the rpm range.
The magnum series cams have a lot of lift with same duration on both intake and exhaust if you have a nice free flowing exhaust system with good exhaust flow on the head.
http://www.compcams.com/Company/CC/c...?csid=151&sb=2
One thing I do like about single pattern cams is a lower overlap period, and more vacuum. Run some by your builder see what he has to say. He'll should be familiar with the flow on the exhaust on those heads.

Just keep in mind that what you gain up top will be given up down low to some degree. No free lunch.
I can tell you from my experience with my Th350 and 3.08 gears that they need lots of torque to get things moving off the line. Then you still want to rev up to maybe 5700 to 6000 for the shift for good acceleration.
I had to go low on the duration due to loss of cylinder pressure because of high altitude. Best case scenario I'm at an equivelent 9.0:1 on a really good cool day with my CR of 9.9:1.
Most of the time I'm probably more like an equivalent 8.0:1 or so due to heat and altitude combined.
Density altitude. Drag racers know about it.

Check out the results on Comps dynos flat tappet vs roller.

http://www.compcams.com/Pages/401/dyno-sheets.aspx

 

Most of the flat tappet cams look like they've got better power curves than their counterpart roller cams.

 

Thought comp owned the Lunati stuff or the bootlegger/thumpr stuff anyway sounds about the same to me

With a 2500 stall I wouldnt do the 274
Those are the pre 89 swirl port truck heads you have?

Try not to get caught up in sound with the cam thing

 

Good observation at first blush, but look closer. I though this too until I really studied the numbers.
Compare the 274 flat to the 276 roller.

274 flat max torque 410 @ 3900 rpm
276 roller max torque 427 @ 3700 rpm
That a significant bit of usable torque earlier in the RPM range.

Look where the torque curve starts on the roller vs the flat. It starts about 15 lb/ft higher and continues up to peak torque
Look at the available vacuum at 800 RPM on the flat vs the roller. More vacuum = better idle characteristics.

The flat had to rev to 5900 RPM to get to it's peak power output.
The roller, 5200 RPM.
That's time and wear and tear and spring control to get to the higher RPM for the flat tappet.

Even on the 264* roller vs the 268* flat tappet I'm seeing some similar advantages on the roller and it's doing it with 12* less duration @ .050.

Look where the torque curve starts on those two. Roller is a good 40 lb/ft higher.
Torque is what a street car needs to get moving. Running through a TH-350 and a 3.08 Rear ratio you are going to want all the torque you can get at lower RPMs and throughout the RPM range.

Check out the torque curve on the 284* flat vs the 288* roller.
The roller is above 350 lb.ft from 3000 to 6100 RPM.
The flat is above 350 lb.ft only from about 3250 to 5800 RPM.
This is with less @ .050" duration.

Study the details and you'll see the differences.