64 273 power increases

I have a 64 273 OEM motor, in a valiant convertable, that I converted to hydraulics, dual plane Offenhauser, Crane 260 cam, 9.5 pistons, electronic ignition, headers, Holly 390 CFM carb, and TCI torque converter.   I want more power so I am asking if anyone knows if 302 casting swirl-port heads will bolt on?   I know that I would either have to modify the intake or buy new because of the differnt angle and size of the bolts.   Also, If this doesn't work, what would?   I called Edlebrock on aluminum heads and they say the cylinder size is too small.  I also have a 69 318 that I have long range plans to install in this car so I would like to maintatin compatibility with that.   Thanks  

Really those '315 'closed chamber heads are pretty decent especially for the displacement.
I say you should port the heads, staying with the 1.78-1.50 valves as your bore of 3.65 would probably shroud the 1.88-1.60 360 valves, Then go with 500-600cfm carb.

The 390 vac sec is a good bolt on choice for the stock 273, but with you combo I bet you would see a very noticeable  HP/top end with more CFM.
302 heads will bolt on to 273-318-340-360, to answer your question.
318 w/ 273 steel crank and 340-360 floating pin rods +your existing top end except w/ 1.88-1.60 valve ported heads would be bitchin.
I would also go back to a solid cam, but not a 'must' to make good power.  
age is no lock on brillance

The cylinder size is too small for the larger heads.  You can put 1.88 and 1.6 valves in your heads.  I have them in mine.  You also would lose a bit of compression with other than 273 heads.  If you also have the bowls cut and do some porting these heads work OK on the 273.
Don  

Install larger valves and clean up the bowls and ports on your existing heads.

The above statement about carb size is correct though, you'd notice a big improvement by going to a 650 cfm carb.  

THANKS GUYS...Good info

I was considering going to a 500 CFM Demon.   I am deathly afraid of overcarbing--good or is there better?

I was also considering trying 1.6 ratio rockers.   Are they worth it or would they cause additional wear?

Is anyone good at estimating HP, what do you think I have now, and what would I have after these mods.  

I can barely get a chirp out of the tires now although it seems to accelerate good.   These desert roads probably don't help either, and I am afraid to bang the 7.25/3.55 too hard either, although I have an 8.25 in the wings.  

600 -650cfm is what I would run, but hey 500cfm will work if thats what you want I mean the factory hp 273 had more carb like 625cfm, but hey.
1.6 rockers are a great idea but don't forget to cut the guides for retainer clearance and depending on how big valve/piston clearance along with capable springs w/correct  installed height.
Under carbing  is easier these days it seams, or is it the lack of real tuners.
The 7 1/4 should hang in there for a while but the axles will eventually go along with pinion bearings. JME  
age is no lock on brillance

I am seeing your and Badfish's point about the carb sizing, and the Holly's are much cheaper.   Do you know which carb, between the 600/650, would give me the most power at 2500 RPM?  

Outstanding on everything in your message.   Do you think going to 1.6 ratio rockers would do the same as larger valves?   If so, would there be any drawbacks?  

A faster opening valve = faster initiation of air=more power across the power band of the original grind w1.5.
600cfm will be fine IMO.
It will be like larger valves but not, cause with 1.6 you will have the valve open more but faster.  
age is no lock on brillance

Would I have to be concerned about piston/valve collision?

BTW, I called Holly Tech and they recomended a Street Avenger 570.   They said if I go bigger than that, I would start loosing trottle response because not enough air would flow and it would not work right.  

CUDADON...What else have you done to your 273 and what kind of horsepower/performance are you getting?  

570cfm? exactly get the 600 already, the avenger is a lean idle circuit POS!
You should be fine with valve to piston clearance using 1.6 on the 260* cam, you'll have plenty of clearance. BTW The rpm range for that cam is kinda weak [w/1.5] you would need more duration at .050 to get it to rpm past 5000 rpm, so DEFINITELY use 1.6 !

If you want to make good amount/to a lot of power with a 273
you better get it to revving it!!  
age is no lock on brillance

If I went with the 570, what would I be missing?   I guess I don't understand what the lean idle circuit does and what affect it has.

Sounds like I could have went with a higher rpm range cam, is there advantages going to 1.6 rockers with this cam over a wilder cam?

My goal, maybe not attainable with this engine, is to make the most power I can at 2500-5000.   I am not interested in spinning to 6000 in particular.  

Why not, you got a steel crank, floating pins, lite lil pistons.  
Any how just put it together w/ what you got & 1.6.
you'll be happy.  
age is no lock on brillance

Do I have to remove the heads to install the 1.6s?   You mentioned above that cutting the guides for the retainer clips was required.

BTW, the 570 CFM seems to work pretty well.   It worked right out of the box and had a noticable improvement in power.