While looking through old Rifle and Handloader magazines for a .50 Alaskan article for the Finnish fellows, I came across an article in Handloader #170, 1994, page 36.

The article by Brian Montgomery is about a conversion to .45/90 on a No.IV Mark 1.

I do not know the pressure of the loads, however the author suceeded in chronographing 2,273fps with 350 and 2,130fps with 400gr. bullets using IMR3031 powder in each load along with Remington 9 1/2 magnum primers.
The author felt these loads are suitable for both No.1 as well as the No.4 rifles as he said he kept them under 45,000psi. I do not know how he came up with his pressure numbers, but in comparring to .45/70 data, he is likely quite correct in his estimates.

He used delrin inserts in the magazine to keep the ammo at the rear and to take up slack for single stack loading. It holds 4 rounds and feeds smoothly, according to the text.

The chart I have shows SAAMI specs on the .303 Brit being 49,000psi, while the CIP rating is 53,000PSI.