Custom ar barrel extension

Hi.I’m new to the forum. I am building a custom ar in 45acp. I am looking at placing a Thompson smg finned barrel on a .458 upper. Does anyone know of a barrel extension with the acme threads that would fit the barrel or someone too modify it to fit. Or do I have the barrel end modified to fit the extension. I am now retired with no access to machines like I use to. I have not found a Thompson style finned barrel that is plug and play. I am also going to fit both the rear and forward hand grips to enhance the look. I tell my wife these are adult Lego kits. (She doesn’t buy into it). Any feedback is appreciated. Thanks,