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 $$$$$$$$$$