Roughly 25 years ago when I had both a NM Blackhawk 41 and a S&W 57 four inch, I set to prove to myself the 41 could handle heavy cast bullets just fine. At the time a myth persisted, rarely heard in recent years, that the 41 Magnum wouldn't shoot heavy bullets well.
I wasn't on the internet back then and had to use resources like the "Handloader's Digest" to source bullets. I found eight different heavy bullets from 250 to 295 grains, some still available and some not. The whole purpose of my testing was to determine what bullet weight and design my guns would prefer so I could have a mold made for the desired bullet.
I tested all the bullets with H110, H4227 and 2400 in both guns. All rounds were fired from a Ransom Rest at a range of 25 yards. The results were nothing short of enlightening. I don't know where the idea a 41 would not handle heavy bullets but it is patently false. Both mine handled them all well and one of the most accurate was the 295 grain Penn.
I was fortunate both guns were dimensionally correct for shooting cast bullets. With the exception of a 260 grain SWC from a company by the name of BRP, whom I think is no longer in business, all the bullets tested shot exceedingly well. BRP's bullets were beautiful but while the rest of the test bullets were shooting 1.5" and less theirs were going 2.5-3". Not bad but not great.
I ended up getting an LBT mold for a WLNGC that drops a 280 grain wheel weight bullet and a little lighter with linotype. A linotype version took one cougar for me at about 30 yards before I moved to Alaska. The effect of that bullet through the cougar's chest was immediate and very final. I carried the Blackhawk for many years as my only handgun before I got caught up in my current Bisley obsession. I carried it all over Alaska with that bullet, cast of 50/50 WW/linotype and never had any doubt it would handle a bear as well as any 44 magnum. I never had to test that but still would go afield with it with no worries.
I know this has nothing to do with the OP's quest for a flattop 41 but my testing illustrated, at least to myself, the 41 mag is a great round. It really isn't deserving of the redheaded stepchild status it seems to have. It will run Sierra's 170 grain HP at a very good clip and handle heavy cast bullets with aplomb.
If your search doesn't turn up a flattop 41 there are several smiths who would be happy to create one for you.