If you mark the cuts accurately and only saw the two sides you can see, it will be straight. With splices you would have three sides, say belly is A, end is B, back is C. Mark it out, saw A and B...you are sort of cutting a triangle part way through the wood. Flip it over, use B to help guide the cut for C. Hopefully that makes sense. Full disclosure, I have not cut out any billet splices of any sort, but the principle of only cutting the sides you can see is something I learned from timber framing classes and is works great with handsaws.