200R4 vs A518 vs Keisler A41 fitment

I've got a 69 Dart, very nicely done restomod-ish, with a 440 and I'm looking to upgrade to an overdrive trans. I'd much prefer an OD transmission instead of the GV overdrive. I've been doing a lot of forum searches, reading and research and I'm finding a lot of inconsistent information. Basically I'm trying to find out which transmission fits with the least amount of floor mods. I've found reference that the 200R4 is a almost a "bolt-in", then other references that say the floor needs reworking, same with the A518.

Another option I've found is the Keisler A41, which claims to be direct fit with only minor work on the torsion bar x-member and flattening out the pinch weld at the firewall. Has anyone here used the Keisler? Silver Sport Transmission sells it with a complete installation package.

I don't mind having some minor work on the x-member done, but I'm really wanting to avoid cutting into the floorpan. So, which transmission can be fit with only x-member work and which ones will indeed need floor pan modifications?