Two diameter GG bullets

[flatsguide]

I am a total noobie when it comes to BPCR loading and shooting, so please hang with me.
I just bought a NIB Browning BPCR in .45-70.
I made a Cerro cast of the chamber and a couple of inches of the bore, the measurements are as follows;
Bore dia. .450
Groove dia. .458
Chamber dia. At mouth. .481
As I'm sure most know, there is no free bore and the lead is a very sharp angle and only .048 long on my rifle.
Bought 200 Starline cases. Thickness at mouth, .011.
Thinking of a bullet weight around 530 gns.

If my math is correct, the case mouth inner diameter, ID, when expanded to the chamber wall will be .459 ID.
I have not Fireformed any of these cases so I have no idea as to the amount of spring back of the case at the neck region.
What I was thinking, is to get a two diameter GG bullet with a base diameter of .458 to .459 about .250 long for the driving bands and the forward portion from the driving bands to where the ogive starts to be at bore diameter +.0005 -.0015. This section would have GG's also. Seating this bullet only .25 inches into the case would give me more powder capacity and the base would not have to be bumped up then swaged down if I understand one of the problems that need to be controlled.
The bullet would be loaded at "as cast diameter".
So, a few questions please.

Has any one have first hand knowledge of this type GG bullet being tried before?

Would a .25 long driving band with a grease groove still have enough material to engage the rifling?

With a 1:30 alloy would part of the bore riding section of the bullet slump enough to provide adequate lubrication?

And last, how would one go about lubricating this style of bullet.

I know it would be easiest to cut in some free bore but modifying the rifle is not an option. Thinking resale later on.
Thanks guys,
Richard