The roller cam in the 400 is out, why is because the depth of the roller lifters is considerably taller than the flat tappet blocks. If you look at a vortec block in comparison to the flat tappet block you'll see the difference in the lifter bore heights. The Stock roller tappets in a flat tappet block would cause the grooves in the lifters not to be lined up with the oil galleys in the lifter bores and oil would be shooting up from around the lifters everywhere and thus causing somewhat erratic oil pressure readings. You would also have to figure out a way to get a spider holder in the lifter valley plus modify the area above the lifter bores to accept the dog bones which keeps the lifters from spinning in the bores. You'll also notice the plastic timing cover and the stamped cover all the bolts don't line up. There's 2 that aren't present on the new school cover compared with the old school. As you said the 400 is externally balanced so you cannot use a 350 balancer and flexplate on it, it must use the 400 externally balanced pieces. If you attempt to use a 350 balancer on a 400 it will quickly destroy the crank bearings. You could have the length of the 400 balancer machined down to match the length of the 350balancer. True flat top pistons in a 400 with Vortec heads would put the compression up around the 10.75:1 to 11:1 figures, way too high for pump gas, especially using the short 5.565 400 rod. You would have to have a cam button on the front of the cam to prevent cam walk, you can buy these from summit or jegs, I believe crane and other companies all make these. They fit in the snout of the crank and is bolted in place by a ring that bolts on to the cam and has bend locks on them to prevent the bolts from backing out. Then you have to set the clearance between the cam button and the timing cover. If it's too tight it won't last long and if it's too loose it will cause cam and lifter damage. If you want your Vortec motor to have more get up and go, then stick with the Vortec block and throw a Scat 1 piece seal internally balanced 3.750 inch stroke crank in it. This way you could reuse the Vortec Balancer and flexplate. Then you could also use the factory cam retainer and you wouldn't have to modify and grind away in the lifter valley to make it all work. If you go this route you would likely come out cheaper with far less work involved. If you were to take this route make sure you get a Cloyes "Vortec" timing set which consists of a large single roller. This set has to be used for clearance purposes.
I've heard there are books out there that show how to use factory roller tappet stuff in older flat tappet blocks but I would think it would be somewhat shakey.