Shumacher Services lists some adapter motor mounts to put a LA where a B/RB was, I believe. But the feedback I've heard about them is that it's not a complete slam-dunk bolt-in situation.
There were 727s that went behind A and LA motors. Usually in HD applications rather than normal passenger cars. I suspect truck/van chassis, too. The LA and B/RB bolt patterns do differ, as might the trans flex plate/crankshaft/torque converter interfaces. There's a reason for these things! Each trans (and variation thereof) is built to a particular engine torque output, so they might be of the same family, but not the same internally. Plus the 360 is an external balance motor, which further complicates things, too.
The 998/999 trans is the most HD version of the 904 family, BUT only the six-cylinder versions have the 2.75 low gear, the V-8s had the normal 2.45 ratio. The lower-low gear planetary AND will completely interchange with the 2.45 ratio. That's how my '80 Newport 360 was done by a local trans shop. On that car, put thing back to the old 2.45/2.76 orientation of old, but with the lower gear in the trans rather than the rear axle ratio. Makes it feel a bit better.
BUT the end result is that you're only considering putting the engine IN, not what it'll take to make it work in the '68 platform! And THAT can be both expensive and time-consuming, at least as much as getting the engine in there, to start with.
Recommendation -- get the engine fixed and detail the underhood area while it's out. Do the rest later. Less headache and THEN you can do the other stuff incrementally while still enjoying the car.
Just some thoughts,
CBODY67