How should I secure spindles on a baserail without a groove?
The plough (or plow, so I have been told it is called) under the hand rail makes for a simple, secure way to attach a baluster  to the rail. If the cut is a little off, the cut is hidden by the sides and the fillets.

For the bottom fit, it only needs to be cut neatly to fit the bottom plate, if that is what you have and secure in place with a few 4D (1 1/2") or 6D (2") finish nails or a couple of trim head screws after pilot holes are drilled. 

Since there is most likely a hole where the metal baluster used to be, each of the 2 nails will go on either side of the drilled hole since the wood baluster will cover the old hole. If a long enough trim head screw is used, say 2 1/4"-3" long you may only need the one screw to hold it at the bottom. The longer screw will span through the old hole and grab wood on the other side.