Those engines are weapons if built right. They are no different to any other engine, just need the right balance of heads, cam, and carb to suit the transmission, diff ratio and expected use. These came factory rated as high as 425hp as an L78 engine, but these were 11:1, solid lifter, 800cfm carb and rectangular port heads.

My guess is you have really low compression. With those large chamber truck heads and flat tops you'll have something like 8:1 compression. The cam is probably too big for that static compression. I'll go and have a look at what I have to help you.

Had a look at a few books and looked at a few sites. Those heads are oval port but seems to be conflict as to whether open or closed chamber. On one site they measured them as 116cc which is open chamber.
If you look at quality piston manufacturer sites like Wiseco, they tell you that a 402 with 15cc piston dome and 118cc heads gives you 8.2:1 at zero deck and 40 thou head gasket. So with flat tops you are going to be near 7.5:1. With that cam no wonder it is making zero power!

You’d have to look at the cam specs but my guess is you’d want at least 9.5:1 static with that cam.

Edited by user Monday, 29 May 2023 7:58:44 PM(UTC)  | Reason: Not specified

Check your piston are actually flat top. Most BBC are dome top. Once you know what you have work out what chamber size you need. And make sure it’s the right cam for your needs. If you only have small dome pistons, with factory closed chamber heads you might be able to get to 9.5:1 and with the right cam it’ll still make good power. What I’m getting at is a different cam will be a lot cheaper than a piston change as well. There are a lot of good books on these to read. Do a bit of planning before jumping in!

I’ll have a look in my shed, there is a pair of large oval port heads in there somewhere I picked up at a swap meet years ago. I think they are closed chamber but have been ported and had larger valves fitted. Will see what they are. I remember the ports are opened up a fair bit and no idea who did them.