since I had a blown head gasket that cracked my head I have decided to do the following.
I want to put some 305 heads on the 307 I know this will raise my compression ratio but how high will It take it up too. because I dont have the cash to pay for high grade gas maby mid but not high. I will also be adding a tbi injection system off of a 305 there both 5 liter engines so I shouldnt have to change the injectors right? also what would be the best size exhaust system for the 307 2 inch or 2 1/2

If you run a thick head gasket and a little bigger cam you may be fine with the middle grade.

305 heads are the best addition to a 307 next to headers.

But . . . I would recomend rethinking the tbi system unless you have ALL the wiring/computer stuff, you know everything is in working order and have done it be4, you will get more power from a tuned 600cfm or even 500cfm carb, and 20+ mpg is acheivable if you are looking for mileage.

As Dragn said, the added compression would alow you to run a larger cam, and a larger cam would bleed off some compression at lower rpms to allow a lower octane gas. I recomend a solid cam. Look at what people run in 283's with power-pack heads for cam suggestions.

I ran 1-5/8 primary headers and a 2-1/4 inch exhaust along with a 500cfm edlebrock and a performer intake on my 307, ran great till i broke a motor mount :D .

Good luck
-Ryan

so what cam do you recomend? Im getting tired of the old setup and thers a parts place down here that sells complete TBI systems for about 150-200
they also have TPI systems for 500. I was gonna get that since gas is getting so high

$500 for a complete tpi setup, thats a good deal.

As for a cam, i dont have time to get deep into it, but isky (www.iskycams.com) has the z-25 solid that i was thinking of off the top of my head. Of course that would be for a 600 cfm carb.

Not sure what i would do for fuel injection, maybee something similar, but ground on a 112 or 114 instead of a 108. Probably shouldn't go solid with tbi/tpi either.

Sounds cool. I'd recommend a standard manifold with an aluminum TB adapter to mount it. The stock TBI manifolds are huge flow restrictors.

Peter

I don't know if this is any good to mention or not, but I would think a Victor Jr. intake and a tbi should work well due to the straighter runners vs the dual plane type intakes???

But you may lose tooo much low end torque??

pdq67