I know I need a full rebuild but can't afford it now, is it possible upgrading piece by piece? I have awful TC shudder and am thinking of just replacing the TC for now or do I need a VB and upgraded internals too? And do I need to triple disc for 450-500 HP?

If the tranny isn't cooked you can go with a TC / VB together. A good TC is worthless without a modded VB to up the inner pressure of the tranny and better flow and clutch holding capability.I ran a single disc BD converter rated for 450 hp / 750 tq at 560 hp and 986 tg and sled pulled quite a lot and did a few drag strip runs and it held up fine for over 100K until I sold the rig. If I would have drag raced more the TC would not have held up but in low range during sled pulling it did awesome.

Call Goerend Transmission they are amazing parts.