A mashup trying to envision using the description of work they wanted done on the M6 would look like.
At the very end of 1943 it was decided to look into modernizing the existing M6 heavy tanks. This involved placing a T26 turret with the 90mm on the existing hulls and the removal of the dual .50 cal hull MG's to be replaced with a .30 cal
This would make an interesting vehicle in the game and could be competitive at a higher BR.
27 December 1943
Activities of General Barnes. Wrote Mr. Beasley the following directive: it is requested that you investigate the method of mounting the 90mm tank gun on the heavy tank, M6 and variations. It is understood that some 46 of these tanks are to be made under the present program mounting the 3" tank gun. It is desired to modernize these heavy tanks by supplying them with the new 90mm gun. At the same time the tanks should be prepared for water stowage of ammunition. It might be found best to remove the entire turret and replace it with the 90mm turret based on the heavy tank T26, with its superior mounting and sighting system. This angle should also be investigated. Prepare an OCM recommending the removal of the 3" guns and the mounting of the 90mm gun in these tanks.
21 January 1944
OCO-D Planning committee meeting. Reference is made to memorandum dated 27 December 1943 from General Barnes to Mr. Beasley reference the mounting of the 90mm gun on the heavy tank, M6. It is desired to modify the heavy tank, M6 series by supplying them with the 90mm gun. To accomplish this, it will be necessary to put an entirely new fighting compartment on the heavy tank, M6. Since the 90mm gun has been successfully mounting in the medium tank, T26, it is advisable to investigate the possibilities of mounting this turret, with it's superior mounting and sighting system, on the heavy tank, M6 also. In view of the facts stated above, it will be necessary to redesign the stowage in the heavy tank, M6 to accommodate 90mm ammunition.
25 February 1944
RAD 213-2 was issued to Baldwin Loco. Works for the preparation of engineering sketches and modification of one pilot heavy tank, M6 to mount a 90mm gun based on the medium tank, T26 turret, and re-stowage of the vehicle to accommodate 90mm ammunition.
10 March 1944
Monthly progress report. The 90mm gun based on the medium tank, T26 turret, is being mounted on the heavy tank series. layout drawings are being made in Development Branch drafting room and are nearly completed. This project was previously reported as being carried out by the Baldwin Locomotive Works, Eddystone, Pa. but since they are unable to undertake the project at the present time, a local facility, the Chevrolet Motor Division of General Motors Corp. has been obtained to do the work. The mounting of the 90mm gun on the heavy tank, M6 will consist of the following:
A) removal of the present heavy tank, M6 turret and replacing it with a turret based on the medium tank, T26.
B) The stowage of the hull will be rearranged to accommodate 90mm ammunition.
C) The two .50 caliber bow machine guns are being replaced with a .30 caliber bow machine gun.
The price proposal has been received from Chevrolet covering the building of a complete T26 turret for this heavy tank as well as for building the stowage arrangement in the hull. Work will commence on this project as soon as work terminates on two projects being built at Chevrolet now, which will be on or about 15 March.
10 April 1944
Monthly progress report. The project for mounting the 90mm gun based on the medium tank, T26 turret, on the heavy tank, M6 series. Has been cancelled by verbal order 7 March 1944 from Chief, Development Branch. The drawings which have been completed in OCO-D development Branch drafting room are being filed for future reference.
Col. Robert J. Icks. stated that such a tank was actually tested, so while they canceled the program it seems the pilot may have squeaked by and been finished.
And they went as far as to give it a designation (at least when mounted on the M6 hull)
History of the heavy tank M6, Declassified - DOD directive No. 5200.0, 27 September 1958
Weapon mounts for secondary armaments, Prepared for Detroit arsenal, Ordnance Corps, U.S. Army
Contract No. DA-20-089-ORD-36713
AFV weapons profile No. 32, By Col. Robert J. Icks
AFV situation reports, British