Ed-your 400 will be 406 after the punch-out (+.030), so it will tolerate a lot more cam than a 350 or even somewhat more than a 383 just because of the cubes to fill. Biggest factors to consider is where do you want your powerband to begin and end, and how much lift will your valvetrain take without too much of a strain. If you want lots of low end keep duration and overlap "reasonable", and lift under about .540" to minimize strain on valvesprings. I chose for my 408 the Crane "Z-Grind" (has higher lift and longer dur. on exh.side than int.side) grind# Z-286-2 / part# 113542. (If ordered as cam + lifters set is part#113541) On a 350 it's supposed to pull good from 3000-6800 rpm, but on a 400/406/408 should be more like 2600-6400, and I had builder install it 2 degrees advanced to drop it about another 100-200 rpm, so it should, "theoretically" be best from about 2400-2500 through 6200-6300. Lift on that cam is .491/.497 so I purchased 1.6:1 roller rockers to add about .033 for total of .524/.530 overall. I was trying to get usable rpm range of 2500-6200 and as soon as I get this engine installed I intend to find out just how well that worked out. My valvesprings are supposed to be good to .600" lift, but I didn't want to go above .550" myself, because I'm looking for longevity as well as performance. I don't want to have to be replacing cam in 15 or 20K! Comp. Cams has a 274* and 280* straight grind that would keep you more in 1800-5600 powerband and cost a bit less than the dual-pattern types. I don't have part #'s handy but I remember they are in their "Magnum Series". Summit Racing has some very similar to these at good pricing. I've been advised by my builder that it's usually best to stay with Comp.Cams or Crane, because their production grinds are done on better equipment than some of the smaller companies- but many people have used other companies cams with no problem. Remember that your valvespring rates and pressures have to agree with your cam choice, or you'll wear the lobes down in short order, or break/bind valvespring(s). Something along the order of 220-230* @ .050", and .480"-.500" lift would probably give you a nice lo-midrange powerband, and be pretty durable too. Be sure to check cam specs. and reccomended spring height/pressures against what is going to be in place on your heads , so those valvetrain parts can have a long and happy marriage! You can always gain about .032-.033 lift by going from 1.5:1 to 1.6:1 ratio on the rockers. Roller rockers can be found for about $130.oo (USD) a set and up, which is reasonable when compared to using "roller-tip" rockers at about $60.oo/set, and they cut down friction and resistance to free-up another few ponies.
I hope these suggestions help you a little, and AGAIN i am sorry to be long-winded. (But HEY-once u spend it,it's gone-so be sure to spend it wisely!)-Jim (408 sleeper)