can you use a MSD Pro Power coil with an MSD 6AL box? I see that it specifies for 7AL & 8... Picked up a coil for cheap.. any reason I cannot use it? This will be in conjunction with a 2 step

Its not designed for street use if you plan
on it

I have had one on my street car for over 10 years. I just bought another one for my other car I'm driving. no problems it says long duration use. that's the 8251 pro coil

You can use a parts-store '90-'91 F150 coil, about $35. Done that on 3 motors so far that see street use with 540-620 HP and it works great. Grab a pigtail for neat wiring & weatherproofing, about $6. I was tuned onto that coil by TechWest, who does the MSD stuff for Cup.
I fab a bracket for them from some 1/2 x 1/2 angle aluminum.