I'm looking to purchase a set North sails Volt's a 4.2 luff 398 ideal mast 370, 5.0 luff 416 ideal mast 400, 5.3 luff 435 ideal mast 430 and 5.9 luff 448 ideal mast 430 these sail set on a constant curve that is in the middle of stiff top and flex top. I already have a 430 powerex constant curve mast and a tip section of a powerex 370 mast (base is damaged).
The 430 will work fine for the 5.3 and 5.9, if I combine the 430 tip with the 370 base I will have a 400 mast, but will it work OK? As in will the longer tip now make the mast a stiff top mast?

Probably Won't work as the curve will be different but you can always try. Longer bottom section may work better instead. I've done this before. Also check that they do actually go together properly.

a few times I mixed a 370 bottom and 430 top to make a 400 and it wasn't too bad

You'll probably end up with a more flexible mast when you combine those two. For a 5.0 and a 4.2 that's probably not so bad.

Ezzy mix and match instructions specify the base should be from the shorter mast and the tip from the longer. For instance I used to do 400 base with 430 top to get a spot on 415.

Thanks for the replies, I thought a longer top would then make it a stiff top as a 430 mast is stiffer than a 400.
I think the Ezzy are more of a soft top sail and mast, hopefully their mast matching works the consant curve sails and mast.