The SDS cap board is a good approach. It works and sounds fine and is reasonably priced. It also fits right into the amp.
I've just used individual caps of either stock values or larger cap values on the first two sections in the amp depending on what was wanted. Usually that means series connected caps for the first two stages at double the final value....so if I want a 110uf cap in the first stage for tighter bass, then series connecting will require two 220uf caps and two 220k 1w resistors in series with each cap. 350v caps work fine and give you essentially a 700v cap for a given stage when wired in series.
The first two stages in the amp need a higher voltage rating, and sometimes the third one does also. I series connect the first two stages with 350v caps of whatever value, then I use a 500v cap for the third stage. I usually leave the preamp and phase inverter cap sections the same as stock because if you change it, it will affect the sound, since those stages are RC coupled. Changing value on the first two cap stages will result in a feel change, as if you go to larger caps, it makes the bass quicker to respond. The main thing however is getting a higher voltage rating to withstand the higher wall voltage these days.
Places like
www.tubesandmore.com have most of the caps and resistors you'll ever need.
Greg