Greetings . . .
The design looks awesome! Especially the second go round with the runners falling downward and closer to the floor of the manifold.
As for the dimensions - I read tech article somewhere that suggested that intake plenum volume should equal the total cubic inches displaced by the engine - for an injected system - and 85% of displacement for carbs (cause that is supposedly the theoretical max volumetric efficiency attainable with carbs which depend on a venturi effect).
A little arithmetic suggests you're right there: 292*.85 = 248 / 25.5 = 9.7 / 3 = 3.24. So a Plenum of 25.5 x 3 x 3.25 seems OK according to that forumula. But when I look at the old Eddie Edmunds plenum designs, he seems to favor width over height.
The question of runner length and cross-section is bit more involved. Here's a tech article on manifolds for the bent eight that has some interesting formulas in it:
Outtakes on Intakes regards,
Keith
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