They have similar designs, but from totally different drawing boards. The LT5 was actually designed using the earlier LT designs for reference by a Mercury Marine engineer, using Lotus heads.The Northstars have always come out of their own stable. The parts are not interchangable. Northstar used the LT5 design, but not in copycat style. They are not the same animal. More like distant cousins. They evolved from that design. As well the LT5 was a very expensive engine to produce, GM only made it for 3 years and less than 7000 were produced. A great many other things stand in the way. Northstars engine use a PCM designed for it, and can't be altered. It controlls every aspect of the engines running and reconfigures 6 times a second as it not only manages everything but checking itself as well. They are disposable and can't be reflashed. If it or one of it'* modules go astray, there is only one fix, replace. The cylinder walls are stainless steel lined and can't even be honed, let alone rebored. Valves cant be reground. And pulling headbolts off LD8'* and L37 usually ends up with snapped bolts. This engine is made to not be modified, not to be altered, and essentially not to be fixed. You run it till it dies and replace.
Very few things can be done with this engine as we have discussed here for years. You can bore the TB, taking your chances, alot have been trashed, it won't allow much, and use a 80mm MAF and translator. By Granetelli $400. Maybe a 6-8% gain with a CAI pipe. You can use a controller that alters signals and optimizes ignition advance and fuel maps, its put out by JetChip. About the same price and never heard of what HP gain might be had by those tweaks. Or Nitrous it and gain 75HP. I choose to nitrous mine. Wish I could use a bigger wet jet, but the engines stumbles with anything higher because you can't alter the timing.
If your rich or just want to blow some money saved up, there are 2 other possibilities. One if to bolt up an STS rear fed turbo to it and hope the mild boost (6-8lbs) doesn't cause the PCM regurgitate (no takers of this I know of yet. Not going to be a big HP gain.
Other is to use a Magneti Custom PCM $2000+ plus the price to rewire and configure/tune. A lot of Hot Rod customizers use them to put N* crate engines into their machines. Mallet cars uses them as well to control the Northstars they have put in Solstices. That gives you a lot more room to be creative.