Help me win this argument 400 in a 72.

You need to shorten your driveshaft to swap from 904 to 727. You need special conversion mounts and expensive headers. The cooling system may need to be upgraded. The 400 will be fast with a mild cam and stock thermoquad. Stock their compression is real low not a lot you can do without changing the pistons. A performer intake will lighten it up a little.