Slant won't drive over 25mph without cutting out

If the engine just suddenly dies, it is likely electrical. If it slows and stumbles first, could be a fuel problem. Check that the rubber fuel hoses on the suction side are firm. Mine was soft and collapsing at the fuel pump inlet in our 1969 Dart 225 during a cross-country drive in 1991. The engine would start stumbling above 55 mph on a warm afternoon. Probably due to getting the then-new ethanol gas. Best to change all rubber fuel hoses to fuel-injection type. Even better is "for bio-diesel" which has a Viton liner, like Goodyear Barricade.