Thinking of switching from M1 single plane to LD340 but LD340 would need to be milled to fit. Is it worth it?

Doug, going from an M1 to an LD340 is a major jump. Problem is you have yet to tell us what you plan for the car.
It’s like wood working- cutting plain Jane 2x4 then you use a 24 tooth blade. But cutting 1x6”oak for trim then you want a much finer blade like 80teeth.

It sounds like the M1 did not work well- my question is “why?” Going to an LD 340 (my fav intake) will also give issues. Both have their qualities and low points. Maybe send us some more info like street and full interior. Race only. Street racing or gear ratio and transmission. You get what I mean. The more info you give the better we can weigh in on this properly. Otherwise “opinion” means just that.
Syleng1