i have been thinking, im in trouble now

[68GMC454]

ok as you know i got a 68 gmc lwb..i have been seriouly thinking that when i do put a motor in it, im gonna do a straight 6..i think a v6 might work, but not sure n that...now my truck is a factory v8 truck.. i have never went with a smaller motor when building anything..what am i gonna need if i do this..motor mounts..the rest of the stuuf should work..it has a 350 turbo tranny in it..
the reason for this is i already got a big block truck that i cant afford to drive, and would rather drive my 68 than look at it, then drive it every now and then when i can spare some gas..

i know i can pick up a straight 6 pretty cheap..but should i maybe consider a v6 out of somethign newer...just wondering...

thanks for anyinfo