Any recommendations for building a '73 340?

All you need is a stock rebuild, substitue the KB pistons which are a lot lighter and better design, internally balance it and run a 318 balancer, and stick a modern cam in it. If the head rebuilding is top quality on modern equipment you will be close to your HP goal. I couldn't get it done for your budget but you might be able to if your shop is cheap, has modern equipment, and talented staff. From experience that won't be the case - but one never knows. Either way the 340 will run much harder than the 273 did. You will spend for exhaust and the factoy manifolding for the 273 may not fit with the 587 heads. I'm not certain on that but I know some have had issues with the stock manifolds and 340/360 heads. If you have the '73 340 intake you can cap the EGR if it has it, then pull out the floor jets, and plug the extra temp switch holes, and run that. It's a pretty good intake on it's own and can support 400hp if the rest of it is good. You can always change the intake later too...