I have no problem loading 70 grains of any BP, (except Fg) in a modern case, (Remington). These are heavier than most other cases.
The only difference between FFg and FFFg, is granual size, meaning the FFFg burns faster. (Avoid using FFFFg in anything other than flash pans or fire works) Sieving your can of powder will also remove the very fine stuff, this has proven to increase accuracy in BPC loads.
I use a 34" aluminum arrow shaft with a funnel attached as a drop tube and trickle the powder in (takes about 10 seconds). The powder is about .375" from the mouth when I'm done, then compress to about .625" from the mouth with a .030" to .065" card on most loadings.
I use a JIM459545C1 mold with 1:40 lead to end up with a 555 grain bullet.
At the present, I am working on putting in a little more powder (BP), upwards of 72 gr., so far no problems, the recoil is getting a little more extreme, but they still fall from the chamber and haven't flattened the primer any more than a 70 gr. charge. I haven't chrongraphed any of the heavier charges yet, but it has changed the point of impact by as much as a foot at 500 yds.
Avoid the use of smokeless under BP until you have a lot more information. Reading BPCR forums will give you a better idea of the different loading techniques and why you might use a smokeless duplex loads in a BP case, you don't need it to ignite BP.
Ray