are 383 and 400 cranks the same?

yes except one maybe forged and the other cast. The 400 is a good bet to be a cast crank while the 383 is a good bet to be a forged crank. This is not a in stone thing.
There is also a balance difference I think between the engines. Internal and external. IIRC

Main and rod bearings along with the stroke are the same standard size for the 383 and 400. The 383 engines had forged cranks until the 1971 2bbl 383 was changed to a cast crank. All 400s should be cast cranks unless they came with a manual transmission and then they should be forged cranks. These are general rules and may not always be true.

for a mild street engine which is better forged or cast? or does it matter. thanks

mild street, doesn't matter

Which motor are you building? If a 400 use the 400 crank, if a 383 use it I do have a forged steel 400 crankshaft out of a 1976 stickshift truck with the balancer, no flywheel or clutch though

Forged is always better than cast.
Both will work.

find a 440 crank and use it...

All B motor forged crank engines are internal balance. All B motor cast crank engines are external balance and use the same imbalance. The 440 cast crank engine also uses the same imbalance.

With its 2.625 mains and 2.375 rod journals, and the 3.375 stroke, the B motor crank should be nearly indestructible. It has a lot of overlap and the short stroke puts much less stress on the crank.

Looking at it in that light it's hard to say a forged crank has any benefit until you hit 6500rpm. As a matter of fact as the cast crank is lighter it may actually be quicker to accelerate.

So IMHO save your money on crankshafts and put it into pistons and if you have any left over, rods. A lightweight forging with the proper compression height is the key to a higher performance engine. I think you can take 150 grams off each piston/pin and then that 383 will really like to rev.

On the other hand if you want a torque monster there's no substitute for cubic inches and the best way to go that route is increasing the stroke.

R.