the 904 is plenty strong for 5-600 I'd run it before buying a 727. But a 4 speed is king. The gearing, tire setup you have is good. Would not mess with it.
as for budget, you can figure 10 is a solid number as the code of things just keeps rising. A stroker crank set up is around 2 grand. Machining about 2K, 2K in labor, there’s 6. Add 2K for heads, 1000 for rockers, lifters, push rods, 500 for a carb, 500 for MSD, 500 for water pump, alt, starter, **** adds up quick….. did not even get to the 4 speed swap yet…