Boomer,
As you will note, in your COW, the 35-40 Maynard case length is listed at 2.060. COW lists the base dia at 0.400", but, Current users of this chambering simply neck down 38-55 cases. Dies are avail from CH4D. 
I, too like the 35 cals. Main modern hunt rifle is 358Win.

Also -I have been sitting on an as new finished blued Lilja Octagon, 10 twist barrel threaded and chambered in 35-50 Maynard to fit a Brg BPCR. This one uses 30-40 Krag brass full length necked up/blown out. May or may not finish this one what with many other projects going in other calibers.....
beltfed/arnie

Steve Baldwin (Baldwin Sights & gunsmithing) shoots a 'modern' (38-55 case) 35-40 Maynard as his BPCR rifle and is a Master-class shooter, so is good out to 547yd on rams. Steve gave me some of his load data if you would like it. Sage offers a 340gr 35cal bullet mold for fast-twist (10") barrels. DZ Arms in OKC has Steve's 35-40 reamer these days.

35 Win is longer than 30 US but can be made from 30 US by stretching them a bit, necking up and fire forming. Nice hunting round with lead bullets (slightly anachronistic) or jacketed (a bit more correct.) Some of the worst brass Bertram ever made was 35 Win. Off-center undersized flash holes. Heads offset from the bodies. Ugly stuff.

I've got a 35-40 built from 38-55-2.08 brass . It was built by Mike Lewis about 10-15 years ago with a 10 twist barrel and it shoots very well. I shoot a paul jones 345 grain NASA bullet in it with 46 grains of Goex .I have a set of ch4d dies to make the brass but don't use them to load them.

This brings up a interesting question! Does BPCR actually allow a 32/40 size case head in a original 35/40 Maynard cartridge or are just ignoring it? Does BPCR have rules conserning cartridges?

It wouldn't be allowable in ASSRA or ISSA as a Traditional chambering.

I've shot the 32/35 Maynard/Stevens, since 1987 and have swaged the case heads for it, because I wanted to have a real original cartridge. I don't think the Traditional class was even started at that time but, original should be "Original".

I look at it this way, if you a collector would think it ok to rechamber a Maynard to the larger case head then it might be ok.

Frank

I know the NRA wouldn't allow the 35-55 Maynard cartridge because of the oversize case head. They allowed the 35-40 and 40-60 because you could find drawings of a small case head on the I believe 1873 model.

I realise this thread is a year old now but.................

Where would I look to purchase a reamer in this 'modern' (38/55) 35/40 ?

I have 38/55 and 32/40 at this stage so figure I'd like to keep my next project simple
and stay with this case. I have a few heavy 357 molds (35 Whelen) so there's a start as well.

Hopefully I can purchase a reamer and have it sent over here to Oz as buying a barrel in The States and having it cut then sent over seems to be a problem.

Any help here would be very much appreciated,
Thanks Ian.