Possible 451 build options

I'd go with the 400, pull the heads off, bowl port/VJ for 2.14/1.81 valves, throw a decent cam in there with beehive springs and call it good. Similar to this build, but I'd use a cam like Comp XE275HL. You'd need the heads machined for valve seal clearance and a real set of rocker arms would be nice. The Erson/Isky/Crane ductile iron stuff is pretty much ideal for this application and used stuff is often reasonable. I got two sets of used Erson's for $250 off C/L locally, for example. But there's often issues with used parts, which is typically why they're being sold. The Mancini aluminum rockers are a good deal for American stuff.

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The 71 440 heads would be 346 heads unless they've been changed (unlikely) and therefore the same, performance wise, as any heads that'll be on the 400. No point in changing unless there's been porting done. A lot of the utility in rebuilding iron heads is dependent on the cost of machine labor or ability to DIY. If you're getting charged $700 to redo the factory heads, a set of Sidewinders or 440 source heads is a better deal; they certainly make more power.

I'd toss the 440 cam unless it something aftermarket. Reconditioning rods may make sense, again, dependent on labor costs. You'd need them bushed for a .990 floating pin if you're going to use them for a 451 for most pistons. More money being spent on marginal parts....
S/F.....Ken M