When I fly fished I used to back feed the but end of a tapered leader through the core of the fly line and use Zap A Gap/ crazy glue to glue it in after roughing it up with fine sand paper. Of course that didn't receive the type of pressure we're talking about here, but it held up really well and I wore out the tapered leaders before the connection ever failed....... I just searched and it looks like some guys are "Chinese fingering" hollow core and using the Zap a Gap. Of course if I'm buying hollow core to chinese finger I might as well just spool it up.The trouble with 400 lb mono is although you could tie a bimini or other knots just fine, it is really big and won't fit through anything other than the absolute largest guides if at all (ie maybe winthrops).. The same is basically true until you get down around 200 lb or smaller diameter lines.
Additionally, the crimps for 400 lb mono are still too big to wind onto most guides.
So the only real splice is a chinese finger of some sort to get onto the thick mono. You could chinese finger hollow core spectra, then tie a bimini or splice a loop in the spectra, bimini the mainline & loop to loop. Or you could use dacron for a chinese finger to the mono and splice a loop in the dacron, bimini the mainline and loop to loop... There are other options too, I do it with the dacron which is easier to chinese finger than spectra, but the spectra is real easy to tie a bimini in so doesn't need a spliced loop.
Oh and braid - it seems abrasion resistant right up until you pull it tight then nick it, then it falls on it's face. Berkley Big Game is where I go for a general use mono up until I'm in the 130 lb sizes.
Can you tell its the winter doldrums? Sitting here at my desk obsessing's about fishing line instead of working.