I got up this morning and dropped a 327 small block in my '53 pickup. It only took about 1/2 hour start to finish. It is and old rusty block sitting in the bed right over the rear axle for traction. I also put some 700X15 mud and snow tires on the rear. It looks like that will be my winter rig because my Yukon broke the block by the starter and I didn't get the Blazer on the road. Sometimes this winter I'll just have to stay home and work in the shop. 

Yes I should have known that , I had a c3 for about 5 years and remember replacing all the battery cables and grounds to fix hard starts.

it was a sneaky little gremlin but its gone now ... just goes to show ,if your main ground goes bad , a jumper cable is NOT going to supply the ground needed for the + amp draw of your starter.   

Glad you found the problem. I thought of ground first but presumed you had new cables already, I almost didn't mention it. I usually check all grounds first (Corvette thing)  , but my luck it's always the bigger ticket items 

^^^ you guys are right.

YUP it was a faulty ground cable... I thought it looked ok , but I even added jumper grounds w/ the 200amp starter pack and it did nothing so I was leaning towards something totally different.

2 brand new monster battery cables all bolted tight and grounded and BAM problem solved, cranks like an animal now.

now just going to slap the trans back on and throw it all back in ….   what a relief , I was thinkin the motor only has 40k what the hell is going on ... you know when you start over thinking    

thank you ^^^

Been there    Bought a running Impala(350) pulled motor/trans and scraped the rest. Needed starter for another project, grabbed that one off. A month later I installed the motor in another car, was in a hurry so I went across the street for a rebuilt starter, slow/dragged. Took it back, exchanged for another. Second new one was slow too. Took back and got the argument that it was my car, said to hell with it. Grabbed an old used one out of my bin.... worked fine! Just cause it's rebuilt, don't mean squat these days. Same with new batteries.

I would make sure you didn't bump distributor and get the timing off a bit first. Then (probably not, but...)  grab another known battery out of one running to try that.  I agree, that shield shouldn't make any diff (1/16" shallower?)   If all that check out ok......try a known starter. I have pulled one off a running vehicle to test before.  Check ground?) Pain in the a$$, but you have to eliminate each component.

That's all I can think of. A 305 shouldn't have a great amount of compression to over come, cold or hot. Good luck!