If you want to go South instead of West, there is a superior shop in Santa Clarita that will build you a far better engine than you will get almost anywhere else.

Take Banshee's advice and don't ask your builder which heads he has. Find the right heads and bring them to the builder, regardless of which builder you choose. There are variations of each of the 4X, 6X and 5C heads that will make a huge difference in how well they could be modified (if need be) to work well on a 400. Bjorn has a set of 5C heads on his 400 which were worked over correctly. As far as I know, they have been a positive addition.

Advice is free ... so, you get what you pay for. Experience isn't free, so if you can benefit from someone else's take advantage of the opportunity.

When you start to put together an engine from parts scattered across different years, there will be a few more details to look for, such as timing covers, water pumps and accessory brackets and pulleys. Even though everything will bolt up from 1967 (and even as far back as 65) through 1979, there will be some combinations that simply won't line up until you match up other parts from the same era.