I need help identifying if I have the correct starter and length of travel.

The truck is a 54 GMC 3/4 ton with a 248.

My starter spins on occasion. When I turn the key again 90% of the time it cranks right over. I have had the starter rebuilt for a second time. The most recent was a very reputable shop and a guy that works on all the old stuff.

I can see some teeth wear on the flywheel on several spots maybe as much as a 20% to 25% worn. It still seems to have plenty of tooth to engage. The gears on the starter are new.

The starter number is 1107109 and a 101 on the nose. I feel the starter number is correct.

The nose is a total of 3-3/8" total from housing to longest point.
The teeth on the starter start at 1-3/4" from the housing and at maximum travel go to 2-1/2"
The flywheel is about 2" from the bell housing.

The flywheel is 3/8" thick.

The math seems correct to me but the starter guy seems to think my nose is too long and the starter gear goes in to far and does not get enough momentum to engage sometimes.The starter guy says it might need to be packed out 1/4" with shims. I did see a hydra-matic starter on eBay recently for a '55 gmc with a 270 engine with a shorter nose?