A couple of things to remember:

First as was mentioned, the intake manifold on a 429 or 460 is going to weigh about 100 pounds. Using aluminum will shave off a bunch.

Second this frail little solid iron bohemoth truck is rated to carry over 1000 pounds of extra weight so an extra 150 poundes above a stock 239 flathead, or Y block isn't going to hurt it. That's probably as much as one of th etwo passengers you will sit in the cab.

I have a 390 FE in my 51 with a C6. It's fine, you arent going to hurt it if you want to pu ta 429 or 460 in it - weight wise. Now space is a different matter all together.