just installed a ho alt from one truck to another now i have slight issuses

[Patrick Saxon]

ok i just got a new truck 2001 gmc sierra 6.0 L i took my mechman g series 270 amp alt off my 1999 sierra 5.3 L to swap on the new truck when i start the truck volts high like 15 and if i rev engine they drop and all cab lights and head lights dim. Once truck is warmed up it seems to be normal voltage and slight light dimming i have big 3 done in sky high ofc and a xs d3400 as starting battery .i have not installed my extra batts yet witch is 3 more group 31s i have not changed any thing with the electrical it has stock sterio ,speakers ,ect ive also noticed i blew my cig lighter fuse/abd port fuse like 3 times without any thing plugged in it .any help would be great