Great! Here is all the information I have on these projects, apart from odd mentions in USMC papers about the need to fund the now EFV:
From War Machine 73 Modern Amphibious & Oversnow Vehicles
Published in 1985
LVTP6
In the early 1950s the Ordnance Division of FMC Corporation, which had produced 11,251 LVTs during World War II, built four prototypes of a full-tracked armoured amphibious troop carrier called the LVTPX2. This vehicle incorporated many automotive and suspension components of the M59 full-tracked armoured personnel carrier that the company was building for the US Army. The four prototypes were delivered to Camp Pendleton in February 1953, and each vehicle was put through 350 hours of endurance testing over all types of terrain and through heavy surf. These vehicles were then modified and further trials were car-red out in 1954, 1955 and 1956, after which the type was declared standard and given the type designation LVTP6. In the end, however, the LVTP6 never entered production. The LVTHX4 was the fire-support vehicle and fitted with a turret armed with a 105-mm (4.13-in) howitzer. The LVTAAX2 was fitted with the twin 40-mm turret of the M42 self-propelled anti-aircraft gun (also fitted to the previously-described LVTAAXI vehicle). A bolt-down 107-mm (4.2-in) mortar kit was also developed for the LVTP6, and projected versions included recovery, engineer and command vehicles.
LVA/LVT(X)
The LVTP7 series was to have been replaced by the Landing Vehicle Assault (LVA), and in 1976 the US Navy awarded conceptual design contracts to the Bell Aerospace Division of Textron Inc., to the FMC Corporation and to the Pacific Car and Foundry Company. The following year Curtiss-Wright was awarded a separate contract for the development of a stratified-charge rotary engine for the LVA. The original US Marine Corps requirement was for a vehicle with a water speed of between 40 and 64 km/h (25 and 40 mph), a land speed of between 64 and 88 km/h (40 and 55 mph), good operating range on both land and water, the ability to carry between 18 and 22 fully equipped troops, and the provision of a turret-mounted 25-mm cannon plus co-axial 7.62-mm (0.3-in) machine-gun. The idea was that two of the companies would be awarded further contracts which would eventually lead to the construction of full-scale mockups. But in 1979 the Landing Vehicle Assault was cancelled and contracts were subsequently awarded for the Landing Vehicle Tracked (Experimental), or LVT(X), to General Dynamics Land Systems Division, FMC and Bell Aerospace. As of late 1984 all three companies had submitted their proposals and two companies will probably be awarded contracts to build prototype vehicles, one of which will eventually be placed in production. All companies have proposed vehicles fitted with a two-man power-operated turret armed with at least a 25-mm cannon plus a co-axial 7.62-mm (0.3-in) machine-gun. The Bell and General Dynamics proposals also have two remote-controlled 7.62-mm. (0.3-in) machine-gun installations above the rear troop compartment; these have been fitted as the US Marine Corps expects that in future amphibious operations the US Marines will have to fight further inland than in the past. Although the LVT(X) will have improved mobility, firepower and armour protection over the current LVTP7 series, it will not have a dramatic increase in water speed: unless there is a considerable breakthrough in hull design (apart from the installation of hovercraft type skirts as proposed for the now defunct Landing Vehicle Assault) there is little scope for further development.