I have seen video of 88's firing while still on the wheels, this was done but was conisdered a emergency measure and meant they were knee deep in the hoopla and didnt want to take the time to completely unlimber.
As far as 75 M61 penetrating a Panther of Tiger I's front armor I wont say it never happened but if it did it was not the norm. Seems there are no shortage of horror stories of 75mm M61 and M72 AP rounds bouncing off fronts of Panthers and Shermans. The phrase I commonly hear is "It was throwing tennis balls at them"
US Army records show that Tank Destroyer crews reported that it was necessary to close to less than 500M to stand a chance of killing a Panther or Tiger and they had a higher velocity gun and better AP ammo.
In Belton Coopers Book "Deathtraps:The Survival of a US Armored Division" ther eis a pic of a Panther that was knocked out at 800m by a M36 Jackson in Normandie. M36 carried a 90mm gun.