I now have two rifles in .45-70, a Pedersoli rolling block with a 30" 1-18 twist barrel and a Uberti high wall with a 32" 1-20 twist barrel. I would like to cast a bullet that would work for both rifles(if possible), what would you fellas recommend as to weight and diameter.

OH, 
Just make sure that whatever (gg) bullet it is, it should
cast at or Larger by say 0.001-
0.002 than the groove dia of each rifle.
beltfed/arnie

I guess I should mention that these will be BP loads and used mainly for hunting with the occasional shooting match, if I can find one within reasonable distance to attend.

If your 1-20 twist is actually 1-20 it should handle a Lyman Postell style bullet of 535 grns, the 1-18 will work with up to a 640 gr spitzer bullet at any distance that you can see the target. I would recommend a Hoch postell nosepour mold 535 gr for both rifles, using black it might be a little kicky if your rifles aren't heavy enough.HTH
LD1

My mold is a old NEI 458-645 GC  only has one grease groove, according to my calipers it is 1.8425 long, I've never shot it with a gascheck on the bullet. For my bbl I needed to size both the nose and base of the bullet.  I'll attach a pic of the mold if I can.

longdistance1,
have you shot that bullet at 1000 yds in any wind at bpcr velocities from an 18 twist barrrel?
I would like to suggest that bullet design would have bad airflow characteristics in the transonic zone, further increasing its instability caused by its great length.
at supersonic speeds it might be ok with smoother drag in the air and spinning faster .
if using black powder the nose bearing surface would bump into the rifling similar to a pp bullet and cause leading unless the bore were prelubed.
keep safe,
bruce.

Bumpup of Longdistance's long bullet ogive with BP will depend, of course, on what alloy he uses. Also, I could picture PPing it and then sizing it to 0.458 or whatever.
beltfed/arnie