I agree with the others. Iíd go with a good valve job at a minimum and the shop may just do that, multi angle at a minimum anyway. Next up, maybe 351W size valves and some minor port work removing sharp edges and bumps if budget allows. The 289 used two basic pistons. The 225 hp A code used pistons that had 4 valve reliefs that displaced about 3CC. The 195 hp C code had pistons with a dish that displaced about 13CC. That 10 extra CC is a full point loss in compression. When you factory advertised compression they didnít include the volume of the head gasket or the 3CC for valve reliefs in both pistons and some other variables that made actual compression less. Stay with pistons having about 3CC volume and your true compression will be about 9.5:1
You want a carb around 500 cfm. Personally I like the Edelbrock and Summit brand. Both have some really good features that make them perform very well which are annular boosters.
On my laptop I have a article bookmarked from one of the on line Mustang magazines. They tested stock exhaust manifolds, K code manifolds and shorty headers on a 67 Mustang with a fresh 289 and 2Ē exhaust system all in the same day, same chassis dyno. The A&C code stock exhaust manifolds made literally the same exact power with in 1hp! The shorty headers made about 10 hp more IIRC. Not sure why both styles of exhaust manifolds made the same power but despite any port mismatch or what ever I would expect to see the K code manifolds to make more power. I wish the magazine explored what the problem was.
Iím going to throw a curve ball at you. Iím cheap, I squeak when I walk. A couple years ago I needed a new motor. I actually have a rebuilt 302 under my bench That I was going to put a set of GT40P heads on (probably the best mass produced sbf head made). After thinking about it, it was actually cheaper for me just to change direction and buy a complete used GT40P motor. Sold the heads I had for $525, bought a GT40P with the trans and EFI stuff for $450. Sold off what I didnít want for $175. So I actually made money and had a very good motor. Bought a used 5.0 cam, used of course for $60, used Stealth intake $80, used 600 Edelbrock carb $125. I recently helped 2nd66 with his car, his old motor took a dump. I got him a complete GT40P with 118k miles for $80! $50 core charge and $100 to pull it or out the door on my trailer for $230.
The point Iím making is donít be afraid to think outside the box. Even at 150K+ miles these engines have a lot of life left in them. You can pick these up cheap, they run really well and even after buying parts needed for a roller cam motor such as flywheel and balancer, itíll still be a lot cheaper then rebuilding your old motor. With a little minor grinding your stock exhaust manifolds will work on the P head with no problems.
One thing great about getting older. A life in prison sentence is less of a deterrence