The 400, due to it's long stroke, is sensitive to changes in the combustion area. You are going to need pistons with a large dish to keep your compression ratio down to a level that will let you run pump gas. I have Edelbrock RPM heads with 70c.c. chambers on my 406 with flat top pistons and can run pump gas without having to back off the timing too much.
The 400 also likes more camshaft than a 350. This has something to do with the stroke and the cylinder diameter, it's really engineer stuff, just remember that you get to have a bigger cam and be happy. That Comp XE274 is going to feel like a stock cam. If you want to stay with a Comp cam check out the XE284 as a mild street cam or step up to a bit more of a dual purpose cam with the XE294. My 406 is a weekend only driver on the street and at the dragstrip and I'm using a Lunati solid lifter cam that is 300 / 308 seat to seat. Part of the reason that I'm running that much cam is to bleed off some dynamic compression. You can read about and download a program from this site that will help you determine what you need as far as compression ratio and camshaft.