Roof/Post transplant?

[MercPowered]

Well congratulations may be in order, I've returned to the Chevy truck fold after many years of absence. I'd owned Chevrolets for most all my life but have been driving Dodge vans the last 10 years.
This changed when my brother heard I was looking for a 3/4 ton to drop a welder on and offered me one he had purchased at auction. He wouldn't include the rebuilt 292 as he has plans to put it in a 50 5 window but said I could get a truck with engine from his friend and use the engine.
Turns out my brothers truck had serious firewall rust issues and his buddies truck smoked, not a good combo. Then his bud offered to throw in a decent engine. In the meantime I had purchased a 93 c3500 6.5 turbo for my welder and though I didn't need it, I decided to buy the small window 1/2 ton w/granny 4 spd for $250 as a project/beater to run around town. When we went to pickup the engine it turned out to be a 250 ci 165 HP Mercruiser marine engine. It took some retrofitting to drop it into a truck but it started on the first turn of the key without a wisp of smoke. Now for the bad part.
In my haste to put it together I hadn't noticed the extensive windshield frame rust which includes the drivers post. My question is, can I do a top/post transplant from my brothers doner truck? If so, where would be the best place to cut the cab? I do have considerable experiance (12 yrs) as a sheet metal welder but using tig. Would wire feed be preferable?
If not feasable, I'll just keep on the lookout for a cab or investigate the possibly using my brothers. And while I'm asking, is anyone familiar with Mercruiser engines? I've read they use race quality bearings and have cams ground for optimal high rpm performance but I'm curious how this will perform in a truck. I haven't had a chance to drive it much as yet.
I'm very pleased to have found this forum and thanks for your observations. Jeff