You cannot use the 4" drums with 4.5 axles even if you drill them. The flange is wider and the drum wont fit flush.
You can have your axles and drums drilled to 4.5 with race studs installed for $200. Cheapest way to go and you won't need new backing plates and brakes for 2.5 width brakes. Leave all your original parts in place.
You will use 8" wheels with a 4.5 back space as measured to the edge of the whleel. so it would be centered 4" in 4" out. when measuring back space you always go to the end of the lip. Lip to lip 8" wheels are 9"s wide.
Been doing this conversion for years and we also do the axles and drums on an indexer. Yes they hold up to 15x10.5 x 27 slicks and do not break the studs out.
Big bolt axles move the flange out 3/8" and the backing plate is deeper for the wider brakes.
Just a heads up. This is all off the top of my head .But pretty sure of the numbers.