I used a 5 1/4 as a scrub also, I don't think I even widened the mouth much, just cambered the blade and went at it. - worked fine til I dropped it It wasn't one of the nice ones though, Handyman or something. Tote was beech I think.
I used the blade, cap iron and lever cap to make a quick wood replacement.
I could have made one of those huge chisel planes you see on ebay now and then Maybe if it had been a low angle jack.
I converted a lost cause #4 into a scrub. The main casting was hopelessly pitted, and the cutting iron was rusty and hammer smashed at the top end. Also, the mouth was fairly wide. I freehand ground a shallow radius on the blade and honed it with the water stones. It's a relatively blunt instrument - no need to hone at 8000 with a guide. Also, the width of the cutter is practically irrelevant. The leading tip of the curve is doing all the work. The rest of the cutter edge just sits there. I'm all for a finely tuned plane, but scrub planes give you no benefit from the time and effort.
The scrub worked fine for several years and some regular use. I picked up a #40 in a box of stuff at an auction, and now use that. The only benefit i can see is that it's a lighter plane - it's noticeably easier to use during long sessions, but the quality of the cut is the same as with the converted #4.