225 Modified Slant

Yeah it is too big. Even a 390 is probably more than the slanty can make maximum use of.
But that doesn't mean you can't run a 650. It kindof depends on the combo.
With 4.10s in the back, and a 3.09 low;it might be fine.
With 2.76s and a 2.66 low, maybe not so much.
If the van is driveable on the primaries, that is to say easily controlled by the gas pedal, and not overly sensitive to throttle opening,and if it is free from bogs or hesitations, and is jettable; then drive it.
In most applications, the primaries alone of a 650 would be large enough to pass enough air for the NA slanty's needs, up to around 5000 rpm. The primary thing that suffers with a too big carb, is low-speed driveability.
A vacuum secondary would be a better choice.
That's why the 318 can accept a big TQ, with the big primaries. The 1.5 primaries are about the same as a regular 2bbl, so for most of the time the teener doesn't even know there is a 4bbl on it. And both the rate of secondary opening, and the amount of that opening are adjustable, to size that carb to almost any engine.