Basically you have a 283 with a 327 crank.
The problem with that is a lot of money to get where you want.
If you want to stay old school and be budget friendly, get a 350 CID 010 4-Bolt block and put that crank in it. It's a "large journal" 2.45 crank and no mods are needed. Go hypereutectic flat tops, bushed rods, after market heads even the DNAs are getting better, or AFR, etc..., Air Gap or Vic Junior Manifold, Holley 650 DP Spreadbore or 750 vac secondary. Depends on how you want to drive. Go to rock auto and get a Delco HEI distributor, Summit (Elgin) cam
[email protected] ratio lift, and
[email protected] duration. M55HV oil pump 5/8 tube.
This will be fun driving and budget friendly 327. Or you can get a 350/400 and do the same.
Or for 5k you are already in the ball park for a 383 stroker.
Or stay as a 307. You might get away with heads but most are for 4 inch bore. So you have to be careful when going past 1.94 intakes on the smaller bores. 307 is only 3.875 inch, so would be very close. 305 Vortec OEM heads would be good or 305 HO 416 or 601 casting which are 1.94/1.5 or 1.84/1.5. If you have machine shop do the heads focus more on exhaust. 1.94 intake with 1.6 inch exhaust is much better than 2.06 intake and 1.5 exhaust.
You can keep the 307 as is.... Upgrade the cam, intake and carb plus a good quality distributor. Have the heads done, but you will have to add hardened seats. Machine shop would know this. Call Engine Tech they probably have a kit for whichever way you want to go and it used to be..., over $500 they give you jobber discount. Or also Rock Auto had complete kits...
However ... 400HP is a big no. HP is Torque x RPM (Simple Terms) so if you could get it past 7000 rpm... Like 11,000 maybe you would get close to 400 HP, but then explode.