040's are good, 041's come with slightly bigger vlaves, wither way for a mild street moter wanting tourque not all out high reving power.....they dont need any welding, just match port them to your manifolds, using your new gaskets, (you'll be using the thick fibre style ones with twin carbs anyway) now with the valves removed.... take out any casting flash down either side of the inlet port with a carbide bur remover tool on a die grinder/fast drill, then smooth the inlet port with a small sand paper style flap wheel, do the same blending in to the area behind the valve seat, you want a semi rough finish on the inlet to aid atomisation (spelling?!?) only advisory on the inlet is to not smooth out that lump 1" down, thats the inlet valve spring base, it needs it for streght.
on the exhaust with the valves removed you want to ease the flow around the big corner, on a stock head you can barely fit your finger between the corner and the valve guide, you can really smooth that bend out, so that you can easily get a finger right down to the valve seat nearly... bigger the better ont he exhaust, and try and leave a polished finish so you need a flap wheel or two with a less corse grit etc...
good luck post up your results.