[72 blackbird]

I've had a 400 w/ ported no.48's in my 'Bird for over 12 years, and had a 455 w/ ported no.64's before that- both engines made nice power, but to be honest the 400 got a little better mpg (13-15 mpg w/ a Holley 750 vac sec. and e-performer) and did launch a little better due to less torque. But what I did like about the 455 was it's ability to kick the tail out at will, and the insane midrange these engines are known for. I had another 400 before the 455 and even that engine blew up a couple of stock TH-350's, so I had one built to handle the power before I swapped in the 455- if done right the TH-350's can even handle the torque from a 455.

The 400 from a '78 will be a no.500557 400, known for having thinner main saddles than the earlier no.979xxxx or 481988 400 blocks- IMO the 500557 block isn't a good foundation for a 400/455 stroker motor- the best choice is the 70-75 481988 block, which will drop into any 70-81 Firebird. But the 78 400 could be rebuilt with the stock crank and will be just fine as long as the block checks out and has no cracks. The 71 455 is stronger in this case, and will make any easy 400+ hp w/ the right parts. Either motor would cost the same to rebuild, but just be sure to ditch those stock cast rods for some 5140 forged I-beam replacement rods and run stainless valves instead of the weaker stock 2-piece units.

Geno

 

[72 blackbird]

That 75 400 block is a thickwall casting- you're good for a 600+ hp build up if you so desire, even with 2- bolt caps. The 5C/ 6x deal could be from the factory, since I've heard of that happening before. You should be able to get another 6x head, but check if it's a -4 or -8 casting first (the number is stamped on a boss next to the valve cover mounting surface).

The cool thing about a 400 w/ 6x heads is that you could run it with the stock 3.75 stroke crank on 87 octane, and then rebuild it later w/ a 4.00 (433 cid) or 4.25 (460 cid) stroker crank and those same heads will produce 8.5-9.6:1 compression, depending on what your heads actually measure out to in cc's.

Geno