It isn't crucial. More of a vanity project. Bike has down tube shifters, but I have some 8 speed Brifters. I thought of putting those on, but don't want buy a new wheel if an 8 speed block won't fit on my existing wheel.
I have a couple of bikes running 8 speed screw on freewheels on hubs that were designed for 7 speed. You can avoid re dishing the wheel by packing out both sides of the hub to the same amount. That will keep the wheel central.You may have to buy a slightly longer axle though. As long as you have some axle in the dropouts and a stout qr you should not have any problems.Sheldon Brown did some trials using a fixed wheel hub with no axle in the dropouts at all. The idea was to try and get some chain ajustment in a frame with vertical dropouts. says it worked well as long as you use an old fashioned strong qr although its one I have not tried.The main thing is the final oln of the hub versus the rear frame spacing. It can and does work though you may have to play about a bit to get it right
Block thickness can be critical when you are doing this so be aware that some freewheels have thick locking rings that can cause problems.Really is just a case of trial and error but as said it can be done