I am currently rebuilding a 454 it was originally going in my truck but have since decided to put it in my 69 chevelle.I have a edelbrock performer plus camshaft adv 292int/302exh,performer manifold,stock heads are the 781 heads 118cc chamber 256/114cc ports.hooker competition headers,the motor also will have the stock bore and stroke.Looking for advice on a good choice for pistons and how much horsepower this combo will make.anyhelp would be appreciated.

As for the pistons, I assume you will be running a 6.135 rod. Below is a variety of pistons offered by KB.

http://kb-silvolite.com/performance.php?action=search&mfg=CHEVY&EngSize=454&RodLen=6.135

I would recommend calling them to see if you can get a piston that will give you ~9.5:1 compression

As for horse power, Here is a motor like yours. These oval port heads have 2.19 1.88 valves which are larger than yours, but it only has 7.9:1 compression and yours should be in the 9.5:1 range (after the new pistons). Those things may get close to offseting eachother. At any rate, the motor made 419 hp and 473 ft lbs. This article is also a good comparason between oval and rectangular port heads in a mild application.

http://www.compcams.com/Community/Articles/Details.asp?ID=-1989283802

Adam

If the engine came with the 781s, it is a low compression engine. Lists at 8.5:1 with 781s, flattops with one valve relief typical. Thing is, conservative clearances, piston far down the hole, CR can be as low as 7.9:1 but 8.2:1 if you are lucky.

You have some parts already....a good combo for a mild driver/cruiser car.

The PERF PLUS Cam is not a lot to get excited about but it is WAY better than the 325 horse, 350, 390, 400 horse hyd 'big' cam of the 60s. My pal that works at Edelbrock said he had that combo, would fry the tires on his DUALIE pick-up! What I'd do is get some more compression via a small dome piston.... My same source tells me you should PORT MATCH the intake runner to your head. The intake runners are for the HD TRK peanut port size. Open the runners up to the just under the size of your larger ports on the 781 heads. I understand this is good for 30 horses......

Do the increase of compression, run a thinner head gasket, good valve job and possible some pocket-porting/bowl work. I think you could easily have 440 horses and 480 ft lbs.
REMEMBER, you are not gonna get great PEAK numbers with this engine combo. You will get great AVERAGE numbers throughout the power band...sure to put a grin on your face!

Do this the easy way and ebay your -781 heads and pick up a set of GOOD closed chamber, -206's that should be down around 96.7 to 98 cc's so that w/ .020" thick shim headgaskets you come up w/ about 9.3 to 1 CR w/ your stock, flat-top pistons so you can run a decent cam.

OR E-brock's 100 cc aluminum "ROVAL" port heads for a good piece of change..

pdq67

PS., I have a bare pair of -206's in my garage now just for another engine like this..

ditch that cam