achieving 250whp on a LS is kinda far if its all motor, but with LS/Vtec its easier to accomplish. Start with your factory ls block (b18a/b18b) and prep it, meaning rebuilt with new internals to handle, depending on your budget or how serious you want it is what parts you will have to decide on. A mildly built is capable of 200whp easily. For example, b18b block with a PR3 head (B16, I do not recommend a P72 head (gsr) because of the clearance from the pistons). Using pr3 pistons or CTRs would def bump compression up to about 11.5 or so which is good for the money. Skunk2 stage 2 cams or PRO series are also recommended but kinda pricey so you can always go with CTRs or ITR cams. Basic bolt ons such as intake, header, exhaust, FPR, etc. with good tuning will help also. Really depends on the $$$$ dude, so if you got it like that then go for it. But just going LS all motor is quite a distance to reach 250whp. Good luck