There is a flow bench manufacturer that mentions in their instruction manual
Norton heads specifically. They suggest using one as an example of what can be achieved. There is more horsepower to be lost than gained when an in experienced enthusiasts tries to improve a Norton head.
IMHO unless you have had a lot of experience porting a T140 head you would be better served replacing the valves with standard diameter Black Diamond units, BD or Rowe USA valve guides and doing a three angle valve job and leaving the ports alone. There is a lot of available extra flow, and HP, reworking the area in and around the valve seat. It is something that can be done in a short amount of time, doesn't cost an awful lot and you will want to do this when you do you cams anyway.
I would also get a new set of STOCK valve springs and stronger push rods (don't be afraid if they are heavier than stock). Alloy Tech make a nice set. You might as well get the low cam lift you paid for and it is almost free HP. The stock springs will conrol the valves at the rpm's you will be turning on the street (7,500) and won't eat up some of the HP you are looking for. If the stock springs serve you now, and you are not floating valves and missing shifts I would stay away from heavier valve springs with he stock cam. You can always put them in later when you install that 'big" cam.
Just my opinion...