[NOPANTS68]

Tommy, the Doug nash (or Richmond 4+1) 5 speed is pretty small compared to say a TKO series trans. It's narrow and the centerline of the main shaft runs high through its body unlike a Tremec. This allows it to slip into most places where M21s used to live.
I just installed one in my '67 Chevelle with zero tunnel mods. I only had to open the shifter hole rearward roughly and inch. I've also recently done the Richmond 6 speed in a '67 Camaro with zero tunnel mods behind a 396. The crossmember had to be reworked due to the length and pad height, but it was pretty simple. The shifter handle and linkage offset have to be played with if you plan on targeting the stock console.
These transmissions can be used behind the LSX blocks as the bellhousing bolt patters are the same, less the upper pass side bolt hole. You have to select the right flywheel and clutch, but the market has plenty of options there. A standard "403" or "621" style bellhousing will work depending on how large of a clutch you need. The depth of the input shaft and centerline to the pilot are the same as well. I'm pretty sure Keisler has a retro deal to make the old trans run behind the LSX block with their own bellhousing. Hope that helps-Good luck.