Not positive on the '59 but your chamber may not be a "D" but one that followed the valves. Not exactly a heart shape but close. I think the compression was about 8-1 which would be great for a blower. Most GMC heads I have used could have .100" milled off without a problem. This may help you get the comp you want. The small port head is great on the street as indicated above. A 1.87 intake and 1.5 exhaust would work well with a blower. I would also o-ring the block/head if you going to run any boost over 10#. Studs for sure. ARP sells them.
I have used a set of Clifford 235 hearders on my GMC's before by splitting the tie bar between to 2 front and then back exhaust ports and they worked perfectly.
Since you will probably be buying forged pistons I would advise you to purchase them in a diameter of an easily purchasable ring size. I would contact Ross, Arias, etc for them as they both work with us 6 cylinder guys. I have a 270 out to 292 3-15/16 and it needs another sleeve maybe 5. The only 270 that could be bored to 4" was a 1940, the first one. If you go to 3-15/16 have it sonic tested in case it has a core shift which I'm sure was my case.
Donnie J is available for consultation by calling Chet Herberts I believe. He ground a blower cam many years ago for me from one of my steel stockers. I'll look up the cam card and see what it was. It is a dual pattern for a 270 flat top piston blown engine I wanted to put in a 50 sedan but it was never completed. I was also going to use the same head you may have. I have all the Comp cam part #'s for using a 1.5" diameter valve springs and Chev V8 length valves if you want or need them. I never did complete the engine but still have all the parts. I believe it has JE pistons/rings and would be about a 277".
I was planning B & M Blower on my own manifold so it would have the stock length snout
Good Luck with the project.
