[DZAUTO]

All of the above is correct, no argument there. BUUUUUUUUUUUUUUUUUUT, since you ask, here is what you CAN do to a 307. First, a little background. A 307 has the same bore (3.875) as a 283 (57-67), and the same stroke (3.25) as a 327. Thus, a 307 is a stroked 283. ALLLLLLLLLLLLLLLLLLLLLLLL 67-earlier Chevy small blocks have the "small journal" crankshaft and the 68-later have the "large journal" crankshaft. The 307 didn't come out until 68, thus, your engine has the large crank. To bore a 307 all the way out (an additional .125 bore) to 4in (same as a 327 or 350) is VERY risky. But, a .030, .040, .060 bore is plenty safe. You can bore it and keep your crank, or, you can bore it and literally drop in a 350 crank. For example, if you bored it .060 and dropped in a 350 crank, it would be about a 338. Pistons for this particular displacement are NOT readily available. BUT, you could do this bore/stroke combination with .060 pistons for a 307 and then use the shorter SB400 rods. Then, just build it the same as you would choose to build any 350 (cam choice, intake, carb, etc).
Cost? Well, it would be more than just a standard rebuild for a 350 (the 350 is the cheapest engine on the planet to rebuild because they are so common), but that is just one of many things that you could do to a 307. If you want to just keep the 307 and not do a lot to it beyond a standard rebuild (which MAY require boring it to clean it up), then a bigger cam, 4bl intake and carb and a set of headers will certainly wake it up noticeably.

 

[DZAUTO]

Just an additional comment.
When I bought the 70 Malibu conv in 98, it still had its ORIGINAL 307, which was VERY tired. I promptly built a SB400 and replaced the 307 and sold it.
Today, I REALLY, REALLY regret doing that!!!
I wish I had rebuilt the 307 with a .060 bore and stroked it with a 350 crank (or, MAYBE, MAYBE with a 400 crank). But too late now. I would have then built is like a very nice 350---------------AND THE CAR WOULD STILL HAVE IT'S ORIGINAL, NUMBERS MATCHING ENGINE!!!!!!!!!!!!!
It was a factory 3sp on the column and I replaced it with a Muncie. I often think maybe I should have searched around for a HD 3sp to replace the original Saginaw 3sp. And instead of swapping the original 10bolt 3.08 non-posi with a 12bolt posi, just added an Eaton posi unit. Oh well, it's water under the bridge now.

 

[DZAUTO]

I too have a 70 malibu. with a numbers matching 307 in it. Bought mine in 99 was ready to pull the trigger and swap out for a 383. Just recently have been struggling with parting with it. I'd love to hear more about what you would've done with your's thanks

Presuming the block and heads were in great shape.
If I had kept the 307 (and regret it now), I would have rounded up a good 350 crank (1/4in more stroke) and had the block sonic tested to see how much bore it would permit. If it would go .060 over, then I would have bored it .040 MAX.
I would have some moderate massaging to the stock heads (especially intake runners and bowls), opened the seats to whatever size valves could be increased (hopefully 1.94/1.6). Added screw-in studs and guide plates. and installed a hyd roller cam kit. Probably flat top pistons (definitely NOT dished) for around 10:1 compression.
This combination would result (depending on bore) about 334cubes.
Induction would be a 66-68 cast iron, YES, I said a cast iron Q-jet intake manifold with a good Q-jet (to maintain a pseudo factory appearance). My choice of 66-68 intakes is because they have the hole up front for an oil fill tube. On valve covers which have a twist-in oil fill cap, I double gasket the cap, then twist the heck out of it until it is fully on and NEVER remove it again. The hole on one valve cover is for a PCV grommet/valve and the hole in the other valve cover is for the tube/hose going to the base of the air cleaner.
I KNOW, I KNOW, I KNOW a lot of people are going with auto trannys---------------------------but auto trannys are against my religion!!!
A high geared rear such as a 3.07 and a wide ratio Muncie would top it off.
NO, the result would NOT be a killer SB400 (or 383), but it would make a very nice performing engine with plenty of torque to turn a 3.07 rear 70-80mph down the Interstate and you would retain the original matching number block/heads.
I REALLY, REALLY like the healthy SB400 in our 70, but there have been many times when I wish I had retained the 307 and built it healthy.
I gave away the original 307 and I don't even remember where it went.

Just as a trivia side note, our youngest son was born at 307pm. I was going to replace the 307 fender emblems with 400 emblems. My wife said not NO, but HECK NO! So, I put a 400 emblem on the air cleaner. Ha, ha (and a personalized license tag).
Now, if you really wanted to trick it, you could add a complete Cowl Induction hood system-----------------it's only $$$$$$$$$$

 

[DZAUTO]

Tom, I love the look of your car. A bowtie would look real slick in that grille though. 😉

I just like the plain look better so I took it out. It's hanging on the peg board over the workbench.
I did not put an SS emblem in its place because the car is NOT an SS.