This will allow a straight run of LMR400 from roof to amp:http://store.rlham.com/shop/catalog/product_info.php?products_id=68864
Also, it only requires a 3/8" hole so that makes things easier.
Yes, the roof is a WAY better choice for an antenna.
At 2m (VHF) the loss of RG58 vs. LMR400 is negligible for that short of a run.
Once you go to 70cm (UHF) and above, the loss starts to climb in the smaller coax sizes.
RG174 is the worst at UHF and the power rating is also low.
Alan K0BG is a frequent poster to the forums and his site is:www.k0bg.com
It would be wise to give it a long look.
I built my mobile setup based on his info.
Even though he would most likely balk at my mobile, it is quite the performer and fills my needs very well.
You get what you pay for and frankly, there is no substitute for a solid and well thought out install.