1970s Building a 76' 500 engine

[_Cap_]

Right now me and my father are about to pull a 500 engine and a 425 transmission from a 76' Eldorado we have. The plan is to send the transmission off to get rebuild and then rebuild the engine ourselves to put in my 75' eldo. Right now my goal is to push around 100-200 more horsepower from stock, so around 500 or 600 crank horsepower.
I know these Cadillac 500s can be built to make some crazy numbers, but mine is still a daily so I'm only looking to increase it marginally
The goal is to do this without going over a mild cam, and without going over 10.5/1 compression.

I was told by taking the heads off of a 472 from the early 70s and putting it on, it gets you to a 10/1 compression and bumps up the hp by around 100. I have no idea if this is true, just some talk from airboat guys who have messed around with them.

Wanted to ask here if anyone had any ideas or experience with this to get the engine to this ideal HP. Because almost all things I can find online have either been returning them to stock, or building them to be 1000 hp racing engines.
Keep in mine it needs to stay at or lower 10.5 compression for all the vacuum systems to operate properly (from my understanding), and is also going to be a daily driver engine, fuel consumption is the main reason I'm not going past an increase of 200 hp.

["Cadillac Kid" Greg Surfas 15364]

Cap,
 The urban myths abound. First of all, the early  (1968-73) cylinder heads have 76 CC combustion chambers and the later (74-76) heads are 120 cc.  If you take the 76 cc heads and install them on a stock 76 500 motor your compression ratio is something like 12.4:1. IF you could find some 120+ octane fuel to run on you might get a 15 HP increase. HP increase require more air flow which means carburetor, intake manifold cylinder head and cam shaft work. There are plenty of parts around to do this, but first of all any aftermarket intake capable of 500 HP would be higher and not compatible with your A-6 AC compressor, nor fit under the hood. Also any camshaft in that HP range would require an increased stall speed transmission converter.
Not all gloom and doom. Replacing the stock pistons with KB flat tops will get you to 9.2:1 compression and a set of large valve, ported 120 cc cylinder heads along with something like a Cad Co 250 cam will get you to the 400 hP range while maintaining a very street-able motor. 
There is another option that is available is supercharging or turbo charging, and the options in that arena are limited only to your wallet.
Point of reference The  motor I built for my 73 CdV would be rated in the 600 HP range, and in order to make it drive-able I've got  3000 RPM stall converter and a 370-:1 Moser 9" rear end. Fuel consumption is about 7 MPG on the road and 3/4 gllon per quarter mile running 109 octane racing fuel
Greg Surfas

["Cadillac Kid" Greg Surfas 15364]

This is what 600 HP looks like

[TJ Hopland]

At a point its kinda a exponential increase of cost for small gains when you first build the motor and then you have the higher costs to operate it like more expensive and maybe harder to find fuel not to mention stuff you will break (like tires to say the least) if you can't keep your foot out of it, also possible unhappy neighbors and tickets.  If you are in and up for all that then heck ya go for 600. At one time that was my goal too but I just didn't have the money for that sort of build.       

You can pretty easily get an honest 300-350 out of these without too much extra expense and still have fun and get in trouble. If you want to spin the tires easier look for an earlier higher ratio final drive.  That 9:1 ish combo Greg mentions is a reasonable place to be.  Probably won't run with a carb on regular gas but could with a decent EFI or maybe a carb cheater.  Mid grade would probably be doable and if the cost spread is like it is in my area save you a decent amount in fuel cost since this is a daily driver.

Also make sure you confirm what you got to work with before you get too deep into things.  Since most of this stuff looks the same on the outside lots could have changed before you got to it.  I have been consulted on more than one project where a 60's car had a 368 in it or someone swapped 425 heads onto a 472.  Find those crank and head casting numbers to know for sure what you have and then get it apart as soon as you can to make sure you don't have damage from sitting. 

Have you found Cad Co?  They have been around a long time and I think possibly the only one still left that specializes in these engines.  Here is their basic ID info page to help you figure out what you got. 

https://cad500parts.com/wp-content/uploads/2019/10/EngineIDPages.pdf

And their site, they can pretty much supply you with as many horses as you can afford....
https://cad500parts.com/     

[_Cap_]

alright, its definitely clear I have a lot more research I need to sort out, thank you for the specs sheet. Depending on cost I may see about doing what Greg mentioned about some bigger valves and maybe a mild cam. I don't think I would go any higher then a mild cam though simply because of the choppy noise

[The Tassie Devil(le)]

It is not Horsepower that gets these cars moving, but torque.

A properly-built 500 will deliver sufficient torque to get you anywhere, and still provide reasonable fuel consumption figures.

My '72 Eldo was rebuilt with oversize 1970 pistons, 76 cc combustion chamber heads, larger valves, a better cam, modified intake manifold with a Predator 5000 Carby (930 CFM), and a good-flowing dual exhaust system with no Cat, or AIR Injection and HEI Ignition.   I think it is about 512 CID now.

I still run all the accessories, and get reasonable fuel economy.  When I want to, it will get going without too much throttle pressure.

Bruce.