350 engine WOT loses power at top end

[Firebirdjones]

Since it happens at wide open throttle at the top of 2nd or 3rd gear that tells me it's a classic example of running out of fuel like I mentioned earlier.

I'm betting you are probably running either a quadrajet or a thermoquad or a edelbrock,,,,all of which have a very small float bowl??

If my guess is right (and I'm betting it is) you probably have a fuel pump getting weak or one that is too small and can't keep up. These carbs are notorious for running dry with long hard full throttle pulls (generally 1st gear alone isn't enough) but by the time you get up into the top of 2nd or 3rd the carb is dry.

Put a full pressure gauge on there and go for a full throttle run in 3rd gear, a nice long pull. Watch the fuel pressure. If it starts dropping below ~4psi you have a fuel starvation issue. Anything below that won't keep the carb full of fuel (unless you run a holley with dual float bowls and vast supply of fuel).
The cause would be any number of things. Weak fuel pump, clogged fuel filter, fuel pump is too small for the application, hot fuel lines that are on the vurge of vapor lock, kinked fuel lines or too many sharp bends, fuel supply line or fuel pickup too small for the application, fuel sock in the tank pickup clogged etc....Start checking all the obvious things I listed before spending money on a new fuel pump.