Bam, depending on how much of the total weight your wanting to pick up, there are several ways to do it, I know where the pilasters are at that support the pit walls, the floor is cemented onto those without any gap under them. I'd take some solid steel maybe two or three inches thick, lay a sheet down across the pit and drive the dozer over it and jack off that plate, or if the machine is wide enough to allow me a gap between the inside of the tracks and the pit wall, I'll just put a piece of steel in the lip on top of the pit wall, then lay a wider one over that that straddles the top of the pit walls, now I'm anywhere from 5 to six inches thick plate steel and jack off of that. I've had to remove transmissions after the dozer was tore apart, then I just jacked up the tracks enough to slide the steel or planks under the tracks and jack off of them.
That's the reason why I poured the floor over the top of the pit walls rather than pour the walls full height first and the shop floor beside it, like many do. The way I did it, I have a wall that's 10 inches thick sitting on a footing that's a foot thick and pilaster that's also full height and there is no gap between that and the shop floor, if I'd shove anything anywhere or break anything, I'd like to see the jack that could do it, I certainly don't have anything that would take that load and bust the concrete even with a point load. The sheets we use are also at least a foot wide.
If for some reason I needed to jack off something lower in the pit, I haven't done it, but I'd take a plate that was narrow enough to fit in the pit itself, and be at least six inches thick, and ideally four to six feet long and jack off the center of it, never needed this yet, but as they say its a plan in theory.
To top that off, I'd take some of the weight off the machine with a use of my shop carry deck crane I can pull in right beside the machine that would be over the pit.
I can't say enough how much I like the lip on the top the pit. I have already put planks over the pit in the lip, six to eight feet the length of the pit and laid a plate of steel over that, slid it in between the tracks from one end or the other and jacked off of that as well. Not sure the most weight so far I've jacked up over the pit at one time, but I have jacked up a dozer cross ways of the pit, and held it up on jacks while I did an undercarriage, worked great, the rollers were hanging over the pit and we impacted them off and new one's back on again. That dozer was over 15 ton, did the same thing with a trencher as well, pulled the roller frames off it and after I was done putting everything on new, put the track frames back on again, that machine was well over 15 ton too.
I built a transmission jack that sits on the pit floor and we do transmission with that for trucks and semi's, only used it a few times and it needs to be completely redone, mainly due to needing something handier to use for that.
Not sure I helped you any, but as of yet, we haven't come across anything we can't jack up or do over the pit, I really like to lower stuff down out of the machines into the pit, lay it on a wheeled creeper I built and roll it out the end, lift it out of the pit with the crane, like transmissions and torque converters if they require a lot of work to bring them out the top.