Shudder While Engaging 1st Gear

[CriticalMass]

I get a pretty severe shudder when engaging 1st gear from a stop and I have to give the engine more throttle than I otherwise should in order to get moving (427/Toploader). It seemed a pretty clear case of clutch slippage to me at first, but to confirm that, I stood on the gas in 4th gear and didn't experience any issues. The shudder/'slippage' is severe enough in 1st that I would expect it to show itself in 4th if that was indeed the problem.

Coupled with the fact that the car only has 20,000 miles on it makes me question the cause. I thought it unlikely that the clutch disc would already be worn out (although they're aggressive miles I suppose).

I checked the motor mounts and there's no sign of any damage (everything is tight as well). I checked/adjusted slave cylinder play and travel. It looks good and clutch fork actuation appears normal.

So my question is does anybody have any other 'easy' suggestions on what to check before I start tearing the transmission off?

Also, assuming I end up replacing the clutch, are there any good references/books out there for a step-by-step guide through the process? I inherited this car and so I'm no expert with it. It'll be my first time dealing with the toploader/bellhousing/clutch.

The car is ~28yrs old. Is there any maintenance that is prudent to take care of while I have the transmission off the engine if I go in to replace the clutch?

Assuming that there is no comprehensive guide out there on the process (I've looked a bit and not found anything) and I just have to start pulling bolts, can the Toploader be removed from below the car, or do I need to remove the trans tunnel and do it from above?

Any help is appreciated,
Thanks.